A management VM is created and started for each of the targets. That machine is named using the pattern mgmt-A where A is the target name.
The corresponding logs can be found in dom0’s /var/log/qubes directory.
For example, say the target was fedora-32, then qubesctl will log into /var/log/qubes/mgmt-fedora-32.log.
For what is worth, if you’re looking for an example of going through the process of writing Salt states for Qubes OS, the following page contains notes I’ve taken when setting up Split-SSH for mysef. The example log file I gave above comes from there.
Corresponding forum thread: Packaging Salt states / formulas for use in Qubes OS