The first is Termux, which provides a gnu userspace atop the Android kernel. This app is pretty old, and well-tested. There is an active and helpful Termux community. But it has some downsides: 1) The version of Termux in the Google Play Store is not the preferred and maintained version, although the Play Store version does work. The preferred version is in F-Droid, but the future of F-Droid itself is uncertain in the light of recent Google policy decisions. 2) Termux does not have access to directories such as /proc, /sys etc, which prevents some gnu/Linux utilities from working and 3) The Termux filesystem layout is very non-standard, so unless a program has been packaged explicitly for Termux, installation will probably be messy. I was able to get most, but not all of the Python packages I use frequently, to run within Termux. I could not get astropy to work, for example. Termux has nice usability features like pinch-to-zoom to change the font size. Termux requests a wakelock, and if you grant the wakelock then the OS will not throttle the app when your phone is locked.

The other option is the relatively recently added "terminal" app. terminal runs a plain-vanilla Debian Linux OS within a VM. Its file system is laid out exactly as you would expect, so if you want to get iPython and lots of libraries, you can just run the Anaconda Python installation script, and it will run unmodified with no errors. Nice! You can also install other nice desktop-style apps like VeraCrypt. There are a few downsides: 1) The OS will throttle the app, and occasionally kill the app, when the app is not actively being used interactively. 2) I have found no way to change the tiny font. 3) It's a Google app, so it might disappear for no good reason, as so many Google products do.

Both of these options work especially nicely on a foldable phone, because then the tiny phone keyboard is much less of an issue. A foldable phone plus the terminal app really is a pocket Linux computer.