Well I am currently in the last grade of high school, I am studying computer science, mostly c++ but not only. I do not know exactly the process behind esea client/ anticheat, but what I do know, and I think I can help you with, is that basically esea, besides vac, can detect cheats in multiple ways(vac does not include these that I am going to mention now, or at least vac is not developed advanced enough in that certain way to spot some cheats):
1) taking random screenshot of your screen so that admins can see if u use any visual hacks
2) fuking up the hole memory in order to search for very specific memory changes, patterns that they anticheat program is advanced enough to find cheats that on vac, that are usually hidden in a more "light way " manner, since vac does not require rocket science protection (not sure but I think this is a smth mm cheats are using : https://en.m.wikipedia.org/wiki/Protection_ring)
3) when u use esea, you agree that u can get random test, that means they can actually access all the data from your pc whenever they want, and they are free to search everything in ur pc.
The reason why esea cheats are so expensive, is that it is a lot of work in order to constantly change the cheats pattern and the way it uses memory, so that it can not be spotted by esea, also it must be without any visuals.
My personal opinion about esea, is bad. I know that now im using this thing, but when i was ge and smgc on cs, i was not cheating. And when i was playing on esea i was not cheating, so i can assure you that my opinion on esea is objective. The reason i don't like esea, is that it makes ur pc run slower, you got no privacy and not only. sry i'm in a hurry sry far any mistakes and poor exprimation