centos 8 yum download only code example

Example: repotrack for modules

Take a look at repo2module project.
#  https://unix.stackexchange.com/questions/567057/download-rpm-and-all-dependencies-on-rhel-centos-8
Assuming you have several modular rpms in ./my-custom-repo/Packages:

modular rpms names like python36-3.6.8-2.module_el8.1.0+245+c39af44f.x86_64.rpm

Run:

cd my-custom-repo
# create traditional rpm repo
createrepo_c .
# generate modules meta info
repo2module  -s stable -d . modules.yaml
# adjust modules meta info to traditional rpm repo
modifyrepo_c --mdtype=modules modules.yaml repodata/