What app would you recommend to learn programming on the go

as @iamavalos said: solearn
it's quite good.
www.sololearn.com

Sololearn is just ok .. idk I spent one day and finished couple of ifs courses .. it gives you the basics .. but I like to do real stuff while learning

I'm personally not a big fan of sololearn. it only teaches you the very basics (if, loops, functions, etc.) and not much more.

most courses there don't even cover file IO or basic design patterns

if you already got those down, don't waste your time on it

@Krokoklemme the emphasis lies "on the go"
sure the capability of sololearn is limited. But i think it's good enough for sitting in a bus or train to satisfy a sudden urge to get an oversight for ex. python.

not an app but you can definitely try gobyexample.com w/ play.golang.org if you were on a bus where you wouldn't have access to your laptop/pc at that time.
myself finding that's a really good way to learn basic concepts. if you want to go further at that time, try using vi/nano with a ssh connection to your own laptop or even NAS. I found that's really helpful when I cannot bring a laptop to my school but an iPad, so I use OpenVPN to go back to the subnet of my home network, and started a server on NAS and using something like 10.6.6.3:8000 to access the server. It's quite an awesome adventure though

Get a book, download termux to practice, get a Bluetooth keyboard for when you can use it.