The build number is 188.8.131.52 RJKEUXM and is 2.9GB big. Of course, it carries the Android 11-intrinsic features but also adds some extra ones coming with MIUI 12. The October 2020 security patch is also baked in, which is rather disappointing given that we are well into December already.
On semi-related note, the Poco X3 NFC‘s Android 11 update seems to be just around the corner as per these recently leaked Geekbench and Google Play Console listing.
One unit has been spotted on Geekbench running on Android 11. And since the X3 NFC is the European version of the handset, this would probably be the first region to receive it with the Indian version getting it shortly after. The Google Play Console entry is also a clear indicator that the software is almost ready to be distributed to the masses.