Sometimes, especially with only an external usb keyboard available, it’s difficult to boot into the integrated recovery mode of macOS.
The normal way
Normally you simply press „Command and r“ at the beginning of the boot up of the Mac. Easy peasy.
The command line way
If that doesn’t work, because for instance if the internal keyboard is fried or such…, you can do it like this:
sudo nvram "recovery-boot-mode=unused"
When you’re done in Recovery mode, choose the startup drive from the top-left menu entry or use this:
nvram -d recovery-boot-mode
Other useful tips: https://www.igeeksblog.com/mac-recovery-mode-not-working/