How exactly do games that use anticheats use a CRC check? how does it actually check lines of code to see if they have been changed or not? also if a user changes some line of code as it's in memory...a crc check will not detect this change correct? it just checks the actual file on the hard disk?

It probably varies from game to game but yeah, i bet you the most common way is they just open the .exe file for read and make a hash.

Are you wondering to be able to protect your own game from attacks like your code being modified in memory?

yes...and how do u check memory modifications in the data segment?

Well,

There is no such thing as perfect security. The goal is to just be harder to crack / cheat than is worth while - and don't be low hanging fruit hehe.

Even if you verify somehow that your code is correct (which you can't cause your code is the code that checks itself... and you are worried that you can't trust it), how would you protect against people altering the value of variables in memory to give themselves more money etc.

If you are making a client / server game, there is no such thing as a trusted client (not really). The solution is to not let the client be the authority about anything you don't want to be wrong.

Anyhow, what are you trying to accomplish with all this?

There may be a simpler, better, more practical way than the way you are going.