External keyboard required. Google Docs keyboard shortcuts on iPhone only activate with a physical keyboard connected.
Command modifier. Google Docs on iPhone uses Command when a Bluetooth keyboard is connected, following iOS keyboard conventions.
Text formatting. Cmd+B, Cmd+I, Cmd+U and other standard formatting shortcuts work with an external keyboard. These are the highest-value shortcuts for editing documents on a phone.
Document navigation. Basic cursor movement and selection shortcuts are available with an external keyboard.
Discovering shortcuts. Holding Command on a connected keyboard in the Google Docs iOS app shows available shortcuts for the current context.