I don’t think we are currently entertaining site selection for the default “Files” area of a channel. Behind the scenes a Teams == one site collection and channels are represented as folders in the primary document library of that site. We DO allow you to pin additional SharePoint sites as tabs. If this is highly desired, I would recommend submitting a user voice for it and socialize the community to vote it up.
(From Richard Dizerega)
Can you answer these?