Legacy hvh is just old versions of csgo from different times periods, the main differences are cheats, and what were the mechanic of the game, since these things were evolving. At first we can distinguish first version of csgo where we had some hvh cheats and it was 2016, then we have the most popular era and it was 2018 and during these times we had fake angels, later fake angles were patched, thus we had 2019 as early desync era, and then later 2020, and 2023 as the most popular version of desync hvh.
If it comes to gameplay there is difference between fake angels and desync era, like you should play a bit differently(hard to explain just give it a try), and at the same time you had different options for anti-aim, and this is main difference between fake angels, and desync era.
For all versions except 2023, you have to download such version from other source than steam, and if you want links just check this website
https://legacy.hackvshack.net/ There is everything what you need to play.
People mainly play this legacy csgo hvh since for some people some mechanics were more fun, or they had particular cheat that was the best for them if it comes to individual preferences, thus they had more fun while playing.