As far as I'm aware find my on iPhone can work even when the phone is off, this is because the phone kinda acts like an airtags where enough information can be exchanged securely.

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.

But it should still work in cases when you lost your phone and nobody else rebooted it, shut it down, or even wiped it, which is still a large portion of cases. I wouldn't call it "completely useless" in that case.

Pretty sure this isn't true. Afaik, you can exclude files from encryption on Android. This is also why you see your custom wallpaper before unlocking the phone.

If Android activates bluetooth after booting, it could - in theory - be tracked with the new Find My Device network.

Not sure how it works on other Android OSs but on Samsung's OneUI the phone won't shut down or turn off wireless connections without a pincode.

That's not how that works. There's special access to some apps that get unencrypted right at boot. That's how your phone can reboot in middle of the night and your alarm will still go off despite you doing the initial unlock. Find my device also has that ability.

See the "ACTION_LOCKED_BOOT_COMPLETED" permission for how it works.

Are you saying it's specifically an issue after restarting ones phone? Just a few weeks ago I was walking my dog and my phone fell out my pocket. I hadn't used it so it was locked and I was able to ring it just fine with Find my Device online. Took me a little while to find the sound, but it located it no problem.

This is only true for the very first unlock after booting.

Yes that's how it works, the device needs to be connected to the net and be able to locate itself.

Some phone makers (if not most these days, idk) don't allow you to even reboot or turn off the phone without unlocking it. So it would need to be placed in a Faraday cage, run out of battery or smashed to not be findable as long as the feature is active.

I didn't know any phones, Apple or Android or Google, allowed potential thieves to just turn the phone off without any passcode or password. That's terrible and this really is a huge weakness.

Yes that's how it works, the device needs to be connected to the net and be able to locate itself.

Some phone makers (if not most these days, idk) don't allow you to even reboot or turn off the phone without unlocking it. So it would need to be placed in a Faraday cage, run out of battery or smashed to not be findable as long as the feature is active.