Since most phones (if not all), use an encrypted filesystem. With such, no service can’t start if the device isn’t initially unlocked after reboot, including Find my device.

Android developers can specify that their apps need to run before the pin is entered, via direct boot mode. This is how alarms still work, even if your phone takes an upgrade overnight, and restarts automatically as part of that process.

I can’t say whether Google’s Find My Device currently does this, but there is no technical reason it can’t.