This always throws me for a loop, where does Bentley live? I’ve seen it referred to as an island and many mentions of “going to the mainland” (i.e. in the university story), but I’m still not sure. Any insight?

Mod K here! Bentley was born and grew up on the island of Minte de Daos, which is part of the chain of islands that used to be California until Dipper destroyed California. For Undergrad he goes to a university in the contemporary USA, near its northern border (southern Wisconsin in current-day terms), and then Graduate school is somewhere a little further southeast of that. 
I hope this helps!