I installed a pen-testing Linux system in Termux. It was a VM/light-VM running from Termux. It was absolutely terrible on my battery and navigation on a phone sucked. I'd like to try a something a little more geared for a smartphone and get a feel for what an alternative Linux smart phone might be like..

Termux wasn't just a terminal, but you could also install packages from the package manager. I'm not sure how that package manager, Termux, and your Android OS all play together. I guess it could be another light virtualized or semi-virtualized system, or it could be installing packages on some level of your Android OS. It wasn't super clear and I didn't dive in too deep, instead following youtube instructions on the specific goal.