[General Topics] zgfg:
Last days I was testing Google's Find My Device (gFMD) vs Xiaomi's Find Device (xFD) on my two Xiaomi phones:

Davinci: Mi 9T, Xiaomi.eu MIUI 12.5.2/A11

Lisa: 11 Lite 5G NE, stock MIUI 12.5.8/A11

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Both gFMD and xFD come as preinstalled system apps

gFMD can be set through Google settings, xFD must be activated through Xiaomi Cloud

gFMD can be installed to every Android with GApps (it relies on Google Play Services)

xFD comes with (stock) MIUI firmwares

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If you have several phones registered to the same Google account (each of those phones may have also other/multiple Google accounts set), with gFMD you can check for each of them

Similarly, if you have several phones registered to the same Xiaomi Cloud account, with xFD you can also check for each of them

For gFMD you must have Google History Location enabled

Of course, both gFMD and xFD require Location to be turned on (although some features like playing sound or erasing data remotely work also.with Location disabled)

Both gFMD and xFD work with Mobile Internet or WiFi

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Precise requirements for gFMD can be found eg here:

https://support.google.com/maps/thread/9580022?hl=en&msgid=9582219

Some screenshots in the guide might be outdated (Google settings may look little different now/with the newer Android version).

But all required settings are properly described - except I was nowhere able to find the Visibility status for Google Play

Note: When I properly set everything, gFMD locating was still failing for me on one phone. Then I disabled gFMD, rebooted, enabled it back and rebooted - gFMD finally started to work then

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Both gFMD and xFD can be used (find location of the phone, etc) through browser (on the PC or the phone, incl. the own phone):

https://www.google.com/android/find

https://us.i.mi.com/mobile/find

gFMD can be used also through that app, but xFD only through browser

Of course, for the real use there is no sense in using the browser on the own phone (except to test do xFMD and gFD work) or in using xFMD app to check the own phone

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With both gFMD and xFD you are supposed to locate your phones.

With gFMD you can also inspect it's history of locations (Maps Timelapse)

With both apps you can force playing a sound on the target/remote device. Also, with both you can erase data remotely and inspect the battery status

With gFMD you can change unlock pin/pattern remotely.

xFD has a similar Lost Mode option (forcing a user on the remote phone to login with your Xiaomi Cloud password to unlock the phone)

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To work properly, both gFMD and xFD must have Autostart enabled and Battery Optimization disabled.

Btw, for xFD, Autostart cannot even be controlled/disabled

gFMD shows notifications on the remote phone that phone is trying to be located and then that phone was located. This might be useful while testing but not in the real case if the phone would be lost or stolen.

Notifications are coming from Google Play Services and must be disabled there: Settings, Apps, Manage apps, Show System apps, find Google Play Services, Notifications, Disable notifications

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Through the tests (while traveling on vacation and now back at home) I found:

gFMD properly locates both my phones in some seconds but Davinci is usually located little faster. Sound can be played even if the remote phone was not located (similarly should work for changing pin/pattern or erasing data but I didn't test)

xFD always successfully locates my Davinci (maybe little slower than gFMD)

However, more often than not, xFD fails to locate my Lisa. On the next try (immediately or after some time), or even after a few unsuccessful attempts it then successfully locates Lisa.

I could not find why it sometimes fails. Eg, I'm testing through browser from the Lisa itself (hence Lisa is not sleeping), and gFMD has just successfully located Lisa, but xFD fails several times to locate Lisa and then suddenly locates successfully

However, you can still play sound when locating fails

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Long story short, I found Google's Find My Device more reliable than Xiaomi's Find Device but I will continue using both (just in case)