For public transit routing, I usually use the app of the local provider. But especially (not only) for Germany, Öffi is an open source alternative for them, that uses their APIs, I think.

FairEmail, email app for Android that has every feature I can imagine. Available on FDroid

OsmAnd, mobile app for navigation and maps. Offline, based on OSM, can do anything. I use it for checking out and bookmarking places, finding POIs and pubic transport stations, routing (especially for bike and foot), measuring distances, and so on. It can also show and record tracks, do car navigation, edit OSM points, and more stuff that I don’t use. Pro/plus/full version free on FDroid