HP Forums

WRPN is a free software emulator for the HP 16C calculator. It is available for Windows, Mac OS, Linux, and Android

http://www.wrpn.emmet-gray.com/

This project has a fork that looks somewhat better, JRPN: https://github.com/zathras/JRPN_Android

The author has rewritten the app in Flutter (https://github.com/zathras/jrpn), but on Android I have issues with that version because it can't be locked in landscape so I won't "up"grade unless that is fixed.