Useful android apps you are using?
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
Merlin - It's a birding app and can identify birds by picture or by their song.
Oh how cool though, I didn't realize they had anything out there that can identify birds by song like that. Is it pretty accurate?
Its really good in North America given that it is from Cornell. Gets probably a bird or two wrong out of hundreds
When I'm in India, its a kinda of a swing and a miss, but they're constantly improving it.
I use BirdNET, I had more luck identifying birds by song than with Merlin. But both are impressive.
I've found auditory ID quite reliable, photo ID is pretty good and questionnaire ID not very.