Reply to Musical Key?

Ok so I'm a noob producer (Into EDM music) & I've been having problems understanding music Keys (Aflat, Bmajor, Cminor for example). Gotten a couple of books but nothings really helped. I understand every song is in one certain key but there are a few things I'm having trouble understanding...

1. When sampling, can you sample something that's in a different key? Does it matter?
2. Does every musical element of a song have to be in the same key? (Vocals, Synths, Piano, Chords for example)
3. When creating in a DAW, how do you make sure every sound your creating is in the same key? Does it matter?

Sorry if these questions sound stupid but I really would like help on the subject, thank you.