Right layers and branches/revs are always defined in webos-ports-setup/conf/layers.txt. Currently we're using LTS Yocto 3.1 Dunfell release through testing (with LTS Qt 5.15) and Yocto 3.2 Gatesgarth through unstable branch. Next stable build will be with Yocto 3.1 Dunfell as well, once we're ready for new release.
The stable, testing, unstable branches in webos-ports-setup are rebased on top of Yocto release branch and the only difference is the branch name in the Makefile. That way if you use e.g. testing branch it will automatically switch between Yocto releases once we switch the testing builds on jenkins. Stable is the same as testing most of the time, but has locked meta-webos-ports and meta-smartphone revisions instead of latest.
Layerman will take care of the checkout of right layers with right revisions - just type make update if you want newer.