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
X-plore file manager.
Local, SD card, FTP, SFTP, FTPS, DLNA, Dropbox, Google Drive, and about a dozen other cloud storages all in the same place with multiple windows you can switch between so you can easily open two folders in wildly different locations and move files between them as if it was all one big filesystem.
Also let’s you host FTP or a webapp from your phone for transfers between other lan devices.