Answer #1:
Discord’s ids (aka Snowflakes) can be used pretty much anywhere, especially in bots
Fun fact: discord ids are just the amount of milliseconds since discord was created, you can use a website like this to see when things were made with their ids
Answer #2:
With someone’s discord id you can get public information about them (You can get their username, tag, profile picture, banner) but not private information (You can’t get their password or token). You can’t do anything malicious since it’s publicly available
Answer #3:
You get their username, tag, profile pic, banner and that’s it.