Hi @alba0266 Glad you have got the new security updates, starting rolling out to unbranded BTU devices yesterday.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 4.0,Android 12 Official Version.
This is actually in the learn more at link from the "what`s new message" when you download your update -> https://doc.samsungmobile.com/sm-g998b/btu/doc.html
Re Security updates info is very often scarce unless a big security issue has been fixed and etc.
Else, re firmware decoding, re CSC, bootloader versions and etc, you can take a look here