From Google support:
https://support.google.com/googlenest/answer/9249713?hl=en
Troubleshoot wired Nest Protect power problems
Wired Google Nest Protects come with long-life backup batteries, so they'll keep on working when you need the most. If your wired Nest Protect stops receiving power from the building's electrical wiring, it will automatically switch to backup battery power.
Even when it’s running off the backup batteries, your Nest Protect (Wired) will stay connected to other Nest Protects in your home. It will continue to monitor smoke and carbon monoxide conditions and alert you in the event of an emergency or Heads Up alarm. It will also continue to send notifications to the Nest app in the event of an alarm as long as the Wi-Fi network is still working.
You'll know your Nest Protect isn't receiving power when the small green light at the top of the Nest button has switched off.