Since Zulip was mentioned, I like to point folks who are interested more to the Architecture overview docs of Zulip. The docs has details on how Zulip make use of Django, PostgreSQL, Redis, Tornado, RabbitMQ etc for building a scalable chat application.

https://zulip.readthedocs.io/en/latest/overview/architecture...

Zulip is Open Source, so do take a look at our GitHub page if you folks want to dive deeper or want to get some hands on experience. We are a welcoming community to new contributors :)

https://github.com/zulip/zulip

Disclaimar: I work at Zulip.