Cities won't build in any way. You will need to build them yourself by manually placing down plots. Out of the box you will be able to build only in your main Outpost and any slave outposts you capture later. You can't build in vassal settlements unless you use the option in the holotape/MCM to retain control of vassals. At that point, of course, you can then manually place plots.
To get raiders to show up at Outposts, you can give Jammer stuff to recruit them (booze, caps and chems), and you can also build a recruitment beacon. I believe chems offer the best chance to recruit more raiders than any other method. If you choose not to take control of vassal settlements, no new people will show up there. Not sure as yet if you can then decide to reraid your vassal to turn it from vassal to slave outposts, but if not, I'd like to make a suggestion.
In short, 1) yes, 2) nothing, and 3) no, not without a recruitment beacon.