It becomes trvial if one uses MapTiler or Mapbox with their own styles and overlays. And even self hosting has much improved since the days of raster tiles.
I'm looking forward to you helping solve this problem then as I have the Android app, but can't use due to lack of downloadable maps.
I have a couple of places in the offline storage, which still works, but if I go to a new place I can't use the app, downloads never start.
MapTiler and Mapbox are expensive.
The maintenance of which is non-trivial.
The fork has maintained their own tiles, so I don't think there's any question of not knowing how to. It's just been inconsistently maintained over time.
With tileserver, it is more interesting. The full one (capable of generating raster tiles on demand) still needs ancient node.js (v10, soon to be eol), which can be a problem. The light one runs OK on the current stack.