It's a kind of visual poetry. Like Blake's poems and images. It's a combination of vision, and sounds and the mind aspect, ideas, thoughts, state of mind. And the aim is to connect to and open out to something we all have in ourselves.
So - they are not really external. But nor are they internal as in "this is me". Because the idea is to connect to something that will open us out to possibilities we never knew we had. So it's not "me" as in the "old comfortable me I'm used to all my life like a worn shoe". So you get an inspiration that is in a way "other". But they are not thought of as independent gods, or independent beings, like your neighbour Joe Bloggs say.
Of course there are beings that are independent of me, like you, like our Joe Bloggs. And there may be beings who have bodies of light and who live for billions of years, who knows. Many Tibetans and Indians firmly believe that such beings exist. Nowadays we tend to think in terms of ETs that live around other stars and in other galaxies, which I think has some similarities with those ideas, in some of the things people suggest for their capabilities.
But these deities are not like that. They represent instead common qualities like for instance compassion, wisdom etc, that we all have. And are "external" only in the sense of "external to my comfortable old usual self where I don't meet anything that challenges me much mainly because I keep ignoring things that point out chinks in the armour I build up to sustain it".
So for instance the Dalai Lama is often said to be an embodiment of Chenrezig, the deity of compassion. But he is no more a deity in the sense of an external deity than you or I am. He will die like us, has no special physical powers, is a weak and fragile human creature like all of us. He doesn't have especial mental powers either, he forgets things, there are things he doesn't understand that other people do, and so on. You are bound to find many flaws in any human being if you look for them.
What makes him special, if he is indeed special, is some kind of special connection with compassion. Similarly to the way that other people like Mother Teresa many think that she also has a special connection with compassion. So if they were Tibetans they would say that she also is Chenrezig to express this feeling they have about her. While others might have other views and not think of her as being Chenrezig, and same with the Dalai Lama and say many Chinese. So when the Tibetans say that he is Chenrezig, they aren't saying that everyone should see him as Chenrezig, they are just saying that he inspires them in that way, just as many people were inspired by Mother Teresa.
If we show compassion without thought for ourselves, in an unbounded open way, we are Chenrezig also.
So going back to the visualizations. The main difference with say Blake's poetry is the way that it is systematized, in the way that many people practice it. They sit and deliberately visualize a particular form. All those details are like that because someone in the past had a vision of compassion, again much like Blake's images, a vivid image arose in their mind that inspired this transcendent compassion (or wisdom or whatever it was). Then they described that vision to others, and then find that this also transmits some of the inspiration that they had themselves from that vision.
So if you visualize Chenrezig as just an exercise in how clever you are at visualizing images, with no connection with compassion and no poetic inspiration at all, that is pretty much useless. It might give you some connection with the possibility that in the future you might get poetic inspiration from that vision, but there is no great virtue in just being good at visualizing a visual form that means nothing to you.
So it has to have that vision / mind / speech aspect to it and involve your whole being or it is rather pointless really.
And - if on the other hand you have a very imperfect vision or none at all, some people are really poor at visualizing anything. But if you have that poetic inspiration, then that's what it is all about. Some people can enjoy poetry, but not have any visual images at all, or hardly any, while reading it. But the images in the poem still work on them somehow. Just they don't see them as clear detailed pictures, they work in some other way hard to explain.
So, if you don't see anything at all visually, but read the text and get a poetic inspiration of compassion, well that's far better than being able to visualize.
And in the end, those who are able to do detailed exact clear visualizations - they have to drop them eventually. Because once you get the message, say of compassion, then they say you have to drop the visual aspect, in the end, to make connection to the inspiration, which is formless.
So if you've spent years, as some do, visualizing some "deity" and can see this deity in every detail as exactly as if they were there as a real person. Or visualize yourself as that deity. But don't have the inspiration that it is meant to inspire. Maybe eventually you get just a glimpse or a moment of inspiration. And eventually begin to connect with what it is all about. But then you may find that your detailed clear visualization gets in the way of connecting to the inspiration once you get the message. So then you have to drop that visualization. But that might be hard to do, if you have spent years building it up, to get the inspiration and drop the visualization. But that's what you have to do.
In the end everyone who works with these visual deities as part of their meditation practice has to drop the visual part of their practice.
In some ways it is easier if you connect with the images and sounds and so on poetically right from the beginning and relate to the inspiration of compassion (or whatever) just let yourself be inspired by them, if you do find them inspiring, and not try too hard to develop the ability to see these forms in your visual imagination clearly. Depending of course on individuals and how you relate to such things.