Why is my iPhone 12 buffering videos?

is buffering happening for videos when connected to internet or with out internet also ?

If it's with internet then check the connection of your network and also try with other networks.

if the problem is with the offline /downloaded /recorded videos then try to take a data back up, erase the device and reinstall the iOS

Buffering is usually down to network speed rather than something on the phone. Are you on your normal WiFi or mobile network? Is coverage good? If you're at home are there others also watching videos or gaming or downloading on their devices?