I'm a python dev, not really sure what's most in-demand right now.
Strictly Android? I like SoloLearn. Pretty stream-lined. I've done a bunch of exercises on it to kill time.
Depending on your patience, exercism.org and termux or an in-browser ide.
Edit: I do agree with all the replies saying you can't code on your phone to an extent.
It's definitely not the best way to start but you can make it work for you if you use Termux to run your scripts.
Nano, the terminal editor is alright. Micro let's you use the touch screen. There's also Acode which is a straight-up android app that you can code in.
To be clear, learning to code on your phone is going to be way tougher than on a computer. There's just more hurdles but it's not impossible. I do a little coding on my phone here and there, mostly for little scripts or small bugfixes. If I'm coding big stuff on my phone, I use something like repl.it
I don't know your situation but if a phone is all you got, then you can do it, just expect to be doing a lot more googling than if you were on a computer.
Self-taught. I have a little discord server with some close friends and one of them is computer illiterate so I started working on a bot with a few commands to help him pirate some stuff and it's grown into thousands of lines of code and I'm constantly adding to it. Runs off a raspi and I'm getting close to my second rewrite.
I started another bot recently that runs an icecast radio station and the bot handles the commands for that, including managing a SQL database for the music library, downloading music and inserting into the DB.
The two bots are a little buggy but they run really well most of the time and most of my effort goes into fixing bugs here and there until I work up the motivation to rewrite them from scratch with all the knowledge I gained since the last rewrite.
Along with a ton of other little scripts and stuff.
I've done a single python class after the first rewrite. Harvard's CS50P (intro to programming with python).
That's the first class I've finished since I dropped out of high school and I learn better with a project I'm heavily invested in. It's been 3ish years since I started that first bot with very little programming background.
I'm now searching for a junior dev job while doing small database projects for a solar installation company.