As in title, preferably open-source, but doesn't have to, I will start:

KOReader - Amazing reader app, and being able to use the same app across various devices, even non-android ones, is pure bliss. Japanese support is pretty good too, which makes me happy. Though.. Sadly no vertical text support :c

Syncthing-fork - Being able to keep a library of books, and some files, and easily keep the shared folder across various devices, is mega comfy. Tachiyomi - Cute manga reader.

NewPipe - Really pleasant youtube client, that i sometimes use to listen to things in the background.

Kaku - An useful Japanese OCR app, that works.. quite decently for when I encounter kanji that I do not recognize, and is not text.

EinkBro - A web browser, that on e-ink devices, is quite comfy. Has few... quirks, but works well on e-ink