Yes, OSM.
OSM are Standard Maps only. There are no satellite maps offered for offline use.
Up to level 17. Although sometimes our tile provider doesn't have level 17 available, and will only build those tiles when requested. Which, does not help users. This is why we are moving to OSM offline vector maps in 2.0.
In Settings you can clear the tile cache, but it will delete all tiles.
Cachly does not do any tile caching other than what iOS naturally does, which we do not know the threshold of.