Most of this is common knowledge I suspect:
Banks are local and not global.
Bank slots can be upgraded with in-game gold or $5 for 10 (currently).
I do not know if bank slots are global or regional?
We plan on having a bank in our player town.
For those that are not builing in our town (not sure why you wouldn't!?), read the following snippet by Numa from SoTA forums:
"There is a weird synergy going on between Kingsport, Ardoris and Owlshead. Kingsport has a boat going to Ardoris, traveling north a short distance from Ardoris gets you to Solace bridge and a rift at Solace bridge takes you back to Owl's head."
Owl's head or Ardoris may be good locations for your starting bank when you are leveling up.
If anyone has anything to add, please comment below.