Reply to Noob questions for Music Theory/Production

Hey guys I've recently been reading about music theory (picked up a book called: Music Theory for Computer Musicians. If you have no formal training whatsoever this book has been great so far. I have no idea how to read sheet music but it incorporates both sheet music examples and sequencer examples for those of us who have only ever touched a midi keyboard)

First question I have is the book emphasizes everything has to be key to sound properly. Can someone elaborate on this. I get that everything must start with the same root note/key but does that mean all your patterns and chords would have to be the same for all your different parts?

Second it seems like music theory largely applies to stuff like keyboards/synths/classical instruments. Does it apply to drums? My first step into production has basically been making drum patterns on Maschine and now i'm just looking to take that to the next step.