What to expect when BB ads a head unit in cubby of '08?

Looking into adding an additional head unit to have USB MP3 storage playback in my '08 base model. The JVC KD-X50BT digital media receiver.

What can I expect if I have BestBuy install the unit? How do I access the new unit's audio mode? Will it take over my CD button on the dash? So I won't be able to play CDs anymore? Which would be okay.

Just trying to figure out how most people access the audio mode of their 3rd party unit using standard dash buttons? Do they take over the radio button, cd button, or what?

 

Would there be a way to use the steering wheel volume buttons once a 3rd party head unit has been added?

Is there some sort of additional wiring harness to integrate the steering controls into an additional head unit?

EDIT: Did some searching and found there's a universal adapter that can automatically learn the buttons and interface with a lot of car models and radio models. Look into something call "Metra ASWC-1". Some on Amazon for $52. Go to the Metra website to check for car compatibility and read your deck's user's manual to check for ASWC compatibility.