I was wondering, for LinkEmailAsync and LinkCustomAsync, does it simply add the associated email and password, and custom id to the current user in the session after checking if those info were already associated to the different user?
Yes, all Link* operations try to attach the given info to the current user’s account, where the current user is identified by the session you’re making the call with.
If that email address, custom ID, device ID etc is already linked to another user account the link operation will fail.