QMK requires a RESET keycode to be added to your keymap in order to reboot into firmware update mode while it's running. Alternatively, depending on the configured options at compile-time, you can either hold ESC as you plug it in, or hold Space+B.

system76's launch requires the holding of ESC on bootup (well, top-left matrix position).

Under normal circumstances there's no "unattended update" functionality built in. Unsure if system76 has modified this behaviour to do so.

(Full disclosure, am a QMK maintainer)