The OS is MIUI. I own the original Pocophone F1 which I use in Asia with local Sim when I am abroad ( in the UK connected to wifi only for emails, WhatsApp etc ). For the price it has been a good phone, still fast with no problems and battery life at 94% after approx two years of light use. It is on MIUI 11.0.9 , security patch 01/06/2020. The drawbacks are :
1. Bloatwares that you cannot uninstall and some you cannot force stop. Recently I keep getting notifications from the Xiaomi music App. I have turned off the notification from the app in the setting but still getting it everyday.
2. Limited LTE Bands , it is ok for EU and some Asian countries but not for the US. I noted even the cheapest iPhones and the £349 Google Pixel 4a ( to be launched in the UK soon) have far more LTE Bands.
I will not buy another Xiaomi phones because of the above if I were to buy an Android phone.