In recent years, the YAML editors for things like Automations, Scripts, even the template page under Developer Tools have implemented support for native select, cut, copy and paste on mobile devices (at least on Android) yet the File Editor add-on has not. It seems odd that it hasn’t since the add-on supports native cut/paste/etc. on desktop devices in browser.
On my phone, I can select text in the file editor in an almost native way but then the small menu with three dots has to be picked to do anything with the selected text. As I have a condition called essential tremors it is nearly impossible for me to pick that three-dot menu and even when my hands are pretty steady it is still not something I’ve had much success doing. In contrast, I can easily cut/paste/etc. on my phone in the other YAML editors that support those features natively.
Overall, the companion app is incredibly easy to use, even for development activities, but for this one issue.