my understanding is that many of them became part of the New Republic, which was part of the reason that the First Order had so many sympathizers in the Senate. presumably a lot of the regional governors stayed in place along with their political structures, just with their military forces defanged. presumably any world that wants its independence from the local governor could petition the new republic for recognition, but i suspect that as the process went on, that became a matter of committees and investigations, and the ex-imperial voices in the senate dragged things out.
i like that the mandolorian series is revisiting elements of the Legends "imperial warlords" concept by having people like Gideon basically operating imperial remnants like the Rebellion used to. so instead of warlords having multi-planet mini-empires, they're now more like real world warlords, where their survival is largely about avoiding notice from the bigger political entities, and their direct power and control is much more geographically limited, though their influence can extend even into regions where they have no direct dominance.
the outer rim region was notoriously lawless and hard to police even under the Empire, and they had a lot more resources to throw at the problem. and even then they barely had a presence in most sectors, focusing on key strategic planets and deploying token garrisons to many of the rest. (this comes up a fair bit in SW:Rebels) though they also liked to use it to hide their secret projects, using that same remoteness from the more densely civilized core worlds to keep things from the bulk of the populace and the senate.
as far as who controls mandalore now.. no idea. a lot of people are speculating Thrawn, as i once did, but i doubt he'd be able to hide that fact, and its clear that very few know thrawn is still around. if the Ahsoka series had come out already, i'd have contunued to think Thrawn myself, but since we're going to be visiting the planet in season 3, i suspect that it isn't him, since we'd have gotten more build up for that.
