The DB app allows you to buy tickets for local transport associations - not sure if it includes everything, but so far I always found the regional tickets there.
The legal difference is that here the DB (app) only acts as proxy, i.e. you don't buy a DB ticket for the local transport but a local transport ticket through the DB (app).
On the other hand if you get a DB ticket across regions it might include local transportation at the begin/destination of the cross-regional route as part of the DB ticket. And this they are not allowed to provide you if your DB ticket isn't cross regional. E.g. if you buy a ICE ticket from one station in Berlin to another station in Berlin they can't offer that. (Most ICEs passing through Berlin stop at least at 2 stations in Berlin).
