> What's your favorite free programming environment for commonly used smartphones?

Termux

  pkg install python
  python
  print('hello')
  ctrl+D

Acode

Termux + neovim

Termux + code-server (vscode-like, accessed through phone browser at localhost)

I don't have a favorite. I do not feel like anyone that I am aware of has made proper investment to make a quality development app for mobile due to the low market demand. While development is better than on a calculator I think they are below my expectations.