Firefox, ppsspp, termux, VLC, Tachiyomi(SY), and KeypassXC/DX are coming to my mind. Probably there are a lot more. These are for android. Although they do apply to desktop except termux and Tachiyomi.
Edit: I haven’t added the various FOSS tools as they don’t really come in “App” Category. Some of them:
Try moon reader pro for android, it got project Gutenberg and feedbooks built in and you can configure your own source also it has a lot of necessary feature like darkmode for books and stuff.