What ID do game developers use for ios devices?

Question

iOS

What information can they get from the user's IOS? (What can be found by the identifier?)
If UDID, then what can be learned from it?
Do developers get an apple id?
What can they learn about the user?

Answer 1

It has long been impossible to get a UDID, large banner networks themselves generate an identifier and save it in the keychain and load it from any application.