Just the default one. I tried some alternative keyboards and they are better in some ways but in the end the default keyboard was enough. Termius provides input of some special keys (e.g. Ctrl, Alt, Esc, Tab, Home, End) so that's another reason why the default keyboard is enough.
Be sure to check the privacy policy on your default keyboard. I've been burned by that before. The default keyboard on my last galaxy phone was sending every single keystroke to a third party and checking their privacy policy showed they used that data for things like market research, guessing at my level of education, building a psychological profile, detecting my interests, etc. and that they in turn shared that data with others.
I switched to AnySoftKeyboard and although the auto-correct/spellcheck is way worse (understandable since they're not collecting every word everyone's typing) the customization and terminal mode are great. I'd occasionally code on my phone in termux (the largest program written on that device was only around 2000 lines) and it did the job.
Nothing reliable that I know of. To have any hope at all of being able to do that with Android you'd need a rooted device. Without root access "your" phone isn't something you can reasonably hope to secure since Google, your phone carrier, and the manufacture all have privileged access to your device while you don't. Even with a rooted device I'd only use an app that you trust. The default samsung keyboard that phone came with out of the box was downright adversarial so at least I got rid of that, but I don't think of cell phones as something I can really secure or trust in a meaningful way.
Just FYI, this goes for all Android users. I believe iPhone has similar capabilities but I have never tried myself.
Your phone likely accepts a physical keyboard. I have a USB-C input, but can use a travel dongle (female USB-C device accepting USB-A) to attach.
I used this a few times to do some very light work when travelling. A good setup is picking up a cheap bluetooth keyboard/mouse combo and using the female input to get both. Many alternatives to this too, e.g. you can also attach a dock to your phone to get all devices your phone has the hardware to accept, and you'd be surprised what it does accept.
