If you are a Buddhist monk or nun, then Buddha's guideline was that you accept a gift of food if it is offered to you, whether it has meat in it or not. Though you can refuse it if you know the creature was killed especially for you or will be killed for you. It doesn't matter also if, e.g. you are offered shrimps or such, many whole creatures in the meal - it's not to do with the number of creatures if you eat a whole one or a part of one or many - but whether it was killed for you.
That's because the main thing is to encourage generosity of others.
And in Buddhism the guidelines about not killing - we have a different reason for this than for most Western vegetarians.
As a Buddhist you are not on a crusade to try and save all living beings from death, in the ordinary sense. Animals, birds, insects, fish etc - they are all going to die anyway. And we can't do anything about that. So - when you take the vow of not killing - it is not particularly to try to stop other people and creatures from killing. You are not for instance taking a vow to stop your cat from killing mice, or to stop a spider eating flies, or a lion killing antelopes.
It is just a way of working on your own conduct. The main objective indeed is to restrain yourself from killing other people, if that is something you might be inclined ever to do. So when Buddhists take the vow of not killing - that is the action which breaks the vow. You also do your best to avoid killing animals and even insects. But, for instance, a Buddhist can be a gardener. When digging the soil you will kill many small creatures surely, can't be helped. But it is not intentional killing and that's not your aim when gardening. When you walk around, breathe even, just to stay alive, then according to modern understanding there are many tiny creatures that go through their life cycles and that, technically, we are killing all the time. But again there is no intention in any of this, so it doesn't even go against the vows.
By taking a vow of non killing then it helps to open your heart and mind to other beings. Like - it is, for many people (but not all, some Buddhist masters of the past have been hunters and such like) - for many people then to restrain from killing animals and insects, as best you can in your daily life, helps you to develop compassion and love towards all beings.
There are plenty of examples of societies and cultures where killing animals is normal and done in a good way. The American Indians, I think particularly, as hunters have a respectful way of dealing with the animals they hunt. (Or at least, did in the past when they were able to live their traditional lifestyle).
As a Buddhist you can't look at them and say that they are wrong, because they kill animals. They just have a different way of working with the world.
It is very much a personal thing also. In Buddhism also, the vow of not killing is one of the five lay vows, which are optional. You have to take these vows as a Buddhist monk or nun, but only some lay Buddhists take these vow. So - it is not as central to Buddhism quite as you'd think. The idea behind it is - compassion and love for all beings, even animals and insects. But taking the vow itself - less so.
You have Buddhist soldiers also in Buddhist countries, obviously, as they have to defend their borders. In the past have been many Buddhist kings and kingdoms. Including King Ashoka, who ruled much of India at one point - though he did all his battles and conquests before he became a Buddhists.
To be a soldier and a Buddhist is a hard path to follow - to try to keep to the teachings on compassion for all beings, even your opponents in a battle. But not impossible and there are still modern Buddhist soldiers.
Of course nowadays also there are many other repercussions to this. So you might refrain from eating chicken for instance, because of concern about battery farming. In which case you'd be okay if it was free range. That would be a reasonable thing for a Buddhist to take.
As for buying meat in a shop - even if you are a Buddhist monk or nun - you could do that as it has not been killed for you specifically. That is except - that if you take the vows strictly - you are not meant to handle money at all. But the Buddha taught that some of the monk's and nun's vows are major and some minor, and you don't need to keep to all the minor rules - there's a technicality here that we don't actually know which were the major and which were the minor rules. But most Mahayana monks and nuns especially treat these as minor rules, and accept the realities that in the modern world you have to do things such as handle money and buy food for yourself. So they would be okay with handling money and buying meat and that is not going against your main vows such as not killing.
So anyway is no problem at all with Buddhists buying meat in a shop for themselves to eat. Westerners often find that confusing. But - the vow is about mindfulness for your own conduct, and not about trying to save all beings from death in the ordinary sense, which is impossible. Nor are we attempting to prevent other humans from killing animals either. We are just working on our own conduct. So, when you live in a society where meat is available as a result of other people who kill animals and then sell the meat for food then it is absolutely fine to eat that meat.
Basically just need to let up a bit on this. It's not as central and important in Buddhism as many may think. Especially for others, that there is no need at all to judge others and say they are e.g. a "bad Buddhist" because they eat meat or even that they killed an animal for food as a hunter (as I mentioned, some Buddhist teachers in the past - widely respected for their realization - have also been hunters, it is rare but does happen).
It is something you decide for yourself. And some Buddhsits do become vegetarian. Many Therevadhan Buddhists especially - they come from warm countries with a largely vegetarian diet.
The Dalai Lama even does his best to be vegetarian. He comes from Tibet originally, where for many generations the people have eaten large quantities of meat, as is natural for a people used to living in a very cold climate with little agriculture (like the Inuit). So he used to be a big meat eater like most Tibetan Buddhists. Now he eats vegetarian when he can. But he accepts meat of course, and he also - tried at one point to be strictly vegetarian but he got ill, and was advised by his doctor that it was because he wasn't eating meat - so he found his body needs some meat just to be healthy.
So that's another consideration - we vary a lot in our constitutions. Some people can manage just fine on a vegetarian diet and don't need meat. Others need to eat meat for health.
And of course, as others have pointed out here, going vegetarian doesn't mean that no lives were killed in order to create your food. Even if you only eat carrots, nuts, and rice or some such - well many living beings were killed in the processes of agriculture used to grow those foods for you. So - from a Buddhist point of view - there isn't really an enormous difference between buying carrots in your local shop and buying, say, a turkey. In both cases many beings died so that you could get your food.
There is nothing we can do about that. But we can develop compassion and love, as best we can, for all beings. And starting with the practical situation we are in, realizing what we can and can't do, is part of that compassion. It is then realistic compassion rather than wishful "pink puffy clouds, rainbows, and bunny rabbits" type compassion. And if you want to take a vow of non killing as part of your practice, that's a personal decision. If you want to become vegetarian likewise. And many Buddhists are vegetarian, but many also are not. And even if you are a vegetarian Buddhist, there may well be situations you encounter, where the most compassionate thing you can do is to accept meat when offered to you.
As for alcohol, then the main problem is not the alcohol itself, or its effects, but whether it then causes you to break your other vows or harm others.
So - that's something you can learn as an individual. And for many a small amount of alcohol (where what counts as small depends on the individual) may be no harm at all, even beneficial.
You can take a Buddhist vow to not take any intoxicants. If you do that, you will probably restrain from taking alcohol at all in your ordinary life. But still there may be situations where you take a bit, where it is part of what is expected - e.g. perhaps at a wedding ceremony or such. And - it also depends on the tradition and teacher - when you take the vow, you might find they advise you that it is okay to still take alcohol in small amounts, in moderation for instance. Because - the main problem is not the alcohol itself but what other things might happen as a result of taking it.
And some Buddhists have learnt to deal with alcohol and even take large quantities without it leading to harmful action. This is the path of the "crazy yogin" where - a Buddhist practitioner may drink a lot of alcohol - and the idea is that it helps them relate to a kind of uninhibited wisdom we all have, but that may be difficult to access because we are so caught up in our ordinary cosy comfortable conceptual divisions of the world. That particular path though is one for very few people to follow. And only done with many precautions and much preparation, e.g. long retreats and such like. For obvious reasons - that it is easy to fool yourself and to think you are connecting with this uninhibited wisdom - when instead all that has happened is that your critical judgement has become impaired so that you don't realize how much harm you are doing. So - to deal with that - it is important that you have a good teacher in the tradition who is able to recognize when this happens, and to help keep you on track.
That is a rare path that few follow. Good to know about it, maybe be inspired by it. Even to make a kind of blessing connection -that in the future some day you might be able to do this also. But not a path for most Buddhist practitioners to follow - and I think in the West particularly, there are far more people who think they are "crazy yogins" in this sense than there are who are genuinely following this path.