To begin, let's open a shell on your Raspberry Pi. You do this via SSH or using Terminal from your Raspberry Pi Desktop.
If you've got a shell open on your Pi, you can run the following:
This will print relevant operating system details, including the version and name of the release.
PRETTY_NAME="Raspbian GNU/Linux 10 (buster)" NAME="Raspbian GNU/Linux" VERSION_ID="10" VERSION="10 (buster)" VERSION_CODENAME=buster ID=raspbian ID_LIKE=debian HOME_URL="http://www.raspbian.org/" SUPPORT_URL="http://www.raspbian.org/RaspbianForums" BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"
There are a few pieces of relevant information here. We can see the Raspbian/Debian version id "10" plus the codename "buster".