Termux.
I love terminals, and to have one on the go is a godsend. I use it for all kinds of things like password storing (my own custom little program[1]), `dict` for looking up some english words, reading man pages during long train rides, managing my VPS through ssh, `ncdu` for clearing up some storage space. I use aria2 cli to download big files often 10 times faster than standard android download system.
If I have to write a small code snippet - I have python and nim installed and always at hand with almost full ide support in neovim. I use completely configurable custom keyboard app[2] with full set of keys[3], so coding on it is a breeze.
[1] - https://codeberg.org/Archargelod/brewpass