[Harry Mao]"How do I add BG music behind a dialogue scene so it actually feels like it's behind not on top of the dialogue.
Eg. a restaurant playing BG music and dialogue happening.
What audio adjustment/effect need to be applied."
You'll probably want to bring it into Audition to play with the EQ & give it a bit of reverb to match the ambient audio of the room. Once you export from Audition & then import into PP, I presume you know how to set the audio level.
[Harry Mao]"Also same question for all other environment folly, what effect need to be applied so it feels like it belongs there"
That depends on the shot. We can't see your shot.
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA