To check out BIND 9 source, type:
git clone https://gitlab.isc.org/isc-projects/bind9.git
This will create a local directory called bind9 which will contain all branches of the source code.
Branch names are of the form v9_X, where X represents the second number in the BIND 9 version number. So for example, to check out the BIND 9.11 branch go into the bind9 directory and run:
git checkout v9_11
Older Extended Release Version branches are named slightly differently, but following the same pattern. To view the latest development version of BIND 9.6-ESV-Rx, you would want to check out branch v9_6_ESV.
To later refresh your copy of the repository, go into the bind9 directory and type:
git checkout BRANCH
git pull origin BRANCH
...where BRANCH is the branch you are interested in viewing.
Whenever a branch is ready for publication, a tag will be placed of the form v9_X_Y. The 9.11.19 release, for instance, is tagged as v9_11_19.
The branch in which the next major release is being developed is called main.
For more details, including information on how to contribute changes to BIND 9, please read https://gitlab.isc.org/isc-projects/bind9/-/blob/main/CONTRIBUTING.md.