This article describes how to set up Mailman3 - The GNU Mailing List Management System on Debian Bullseye (Debian 11). The documents of Mailman3 are pretty... abundant, but honestly pretty confusing too. I won't nominate it to The Nobel Document Awards.
Anyway, here is my way to build up a reliable mailman3 system on Debian 11. This article is a great help, but still there are something you need to keep in mind.