How to identify 'main' ubuntu repository apps

You don't need The Force,

Look for the Source,

It's in the package's Details section, of course.

Now to ride away on a horse.

MAIN:

UNIVERSE:

Use the show option of apt:

show (apt-cache(8))

Show information about the given package(s) including its dependencies, installation and download size, sources the package is available from, the description of the packages content and much more. It can e.g. be helpful to look at this information before allowing apt(8) to remove a package or while searching for new packages to install.

apt show <package> | grep 'APT-Sources'

For example:

$ apt show kate | grep 'APT-Sources'
APT-Sources: http://ar.archive.ubuntu.com/ubuntu focal/universe amd64 Packages