I was happy to find out a few weeks ago that it's possible to build GNU APL on Android within the termux environment (a kind of Debian container that runs inside Android). I forgot to capture the exact configure options I used, as I was running out of storage on my phone so I had to remove the SVN build tree afterwards; but it wasn't too strange. If anyone's curious I can rebuild again and log what I had to do ( I think I had to edit one #include statement somewhere, but I forget ...)
However I was stumped trying to find a good way to input APL as the only APL keyboard I could find on the Play Store was a horribly-outdated thing that didn't work at all.
Happily, I found out tonight via a tip from @Adám on
https://apl.chat/ that there's a fork of the "Hacker's Keyboard" with an APL language/layout and it works really nicely!
The 5-row configuration gives all the APL symbols as expected with long-press of each key, for a great mobile APL scratchpad!
-Russ