On Android you can simply run vanilla llama.cpp inside a terminal, or indeed any stack that you would run on a Linux desktop that doesn't involve a native GUI.
Yep, termux is a good way to do this. Llama.cpp has Android example as well, I forked it here GitHub.com/iakashpaul/portal you can try it with any supported GGUF/Q4+Q8 models
There's an app called Private AI that will let you run models locally on Android. It has a few smaller models available for free to try it out, but the larger models like Llama 3 (or the option to use your own downloaded models) require a $10 unlock purchase.