Community templates are to be installed using qvm-template - kali,
kicksecure, etc. Other templates may be made available from other
repositories - as mine at https://qubes.3isec.org/Templates/
man qvm-template covers the cli.
There is a GUI, qvm-template-gui which is intended to be obvious in
use.
Partitioning, and package sources and choices will be configured for
you as part of the package.
If you are asking about other possible templates based on Debian, (or
Fedora, or Arch) , these can be built by modifying the build configuration
in qubes-builderv2. I posted about the method a long time back for
qubes-builder, but the method is the same - identify what is needed for
the template, and add that in to the build configuration. The standard
process will take care of partitioning and qubes* packages.
If you are asking about just any other possible template, it’s
possible. There is specific advice here
If you just want a template without integration (e.g. networked but no
qvm-tools), that’s pretty simple - create a root.img by installing in
to a raw image, and then use that to create the template package.
Again, this can be done using existing build-template-rpm tool. I’ve
done this for assorted *bsd , android, slackware, tinycore, as a quick
means of distribution only.
I never presume to speak for the Qubes team.
When I comment in the Forum I speak for myself.