Best resources for learning how to build a forum back end?

I believe you are asking to use phone as backend? If its ios, its over because you cant.

But if its Android, I recommend Termux which gives a Linux based terminal emulator and you can install Ubuntu in it for further support.

You will also be needing a reverse tunnel to make it connect to your remote frontend application.

i recommend to only learn only as running heavy resources already heats up phone, and yours old too.

and for your existing projects, try FlashBB, its python based.

good luck.