The Device Tree Overlays branch matches perfectly with the Kernel branch, which you can find at Kernel Version Information. In other words, for upstream based modules, the branch is master , and for downstream based modules is toradex_5.15-2.<n>.x-imx .
As you are using Apalis iMX6, our images (Reference Images and Torizon OS) for this module use upstream kernel. You can get this information from the Kernel Version Information referenced in excerpt above.
The branches you shared in your initial message are meant for downstream kernel. (kernel downstream 5.4 → BSP 5, kernel downstream 5.15 → BSP 6). Using upstream kernel is part of Toradex’s upstream-first policy. You can read more about it in here: Upstream First - Toradex Mainline Kernel Support is a Reality
) our documentation to have the necessary information more easily accessible.