We don’t have direct evidence of Europa’s interior yet, unlike Enceladus where we can find out about the ocean through measurements of its geysers. So ideas are based on modeling it instead, and indirect evidence.

Its icy crust over the ocean may be as thin as 10 km deep. Improved detection of tides at Europa with radiometric and optical tracking during flybys Though it may be thicker. 15 to 25 km thick. Europa FAQ

It has intriguing chaotic terrain which may be caused by hot water plumes rising up through the ice. If so, it might have liquid water quite close to the surface.

It has hydrogen peroxide on its surface Ingredient for Life Common on Jupiter Moon Europa The oxygen is created by Jupiter’s ionizing radiation dissociating water molecules in the ice. Eventually over geological time, large amounts of oxygen should find their way into its oceans. It also probably has free hydrogen to balance the oxygen in it’s water. NASA just found even more evidence that Europa could host alien life

As for salinity, pH etc, I don’t think they know much yet. One of the things to find out in the upcoming Europa mission, if it goes ahead. Yes! NASA is planning a mission to Europa, one of the best candidates for alien life