Forgive my ignorance but does Debian (and other distros) maintain forks of all their software packages? For example, how would an hypothetical mailman "port to

Forgive my ignorance but does Debian (and other distros) maintain forks of all their software packages? For example, how would an hypothetical mailman "port to python3" happen? Would Debian maintain its own fork or would you contribute directly to the GNU mailman project?