Questions about ICE

Maybe this is common knowledge already but I have 2 questions regarding if someone was to get stopped by ICE I’d love to get more info about:

1. ⁠how do they choose who to stop? I’m assuming they have to have more probable cause than “they look brown” right? If so what are they going off of? Are they sting ops?
2. ⁠how does one prove their citizenship? I’ve heard that a drivers license won’t prove citizenship and most people don’t carry around birth certificates so how are they distinguishing the illegals from the citizens before throwing them in the truck?

Just overall curious how this is different than the Ann frank situation just with brown people