If you've got a shell open on your Pi, you can run the following:
cat /etc/os-release
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".