In the past (i.e. when I had an Android device), I installed a Debian
“chroot” inside the Android system. That means you can basically install
any console application from typical GNU/Linux distributions – you use
the Linux kernel from Android + the GNU &al tools from Debian. The
downside of this approach is that you need root access to your device.
Inside this Debian environment, I installed Emacs and OCaml (from the
package manager), opam2 (from the official download site), and compiled
Coq using opam2.
There’s also an alternative without root access requirement, where you
basically install Debian inside an App. E.g., have a look at Termux: