I have enjoyed participating in several Discourse communities.
https://discuss.codecademy.com/
Codecademy offers people an opportunity to learn programming languages, such as Python, JavaScript, C, C++, C#, Java, and more, as well as markup languages, such as HTML, CSS, and others used to create web content. Their Python courses are particularly strong, and include instruction on AI and machine learning. Their very active Discourse community currently incudes over half a million learners. I was an active Moderator there starting in 2015, and enjoyed it immensely.
The Python Discourse community of over 15,000 people serves as a hub of communication for developers and maintainers of the Python language itself, as well as programmers from the beginner to experienced level. Learners and all have an opportunity to interact with each other, including developers and those at the center of the Python community. The inventor of the Python language is among this community’s active members.
Processing and a related set of languages enable artists to use programming to create art. The Processing Discourse community of over 10,000 is a fun place to discuss this set of languages and to display and write about the art itself.
OpenStreetMap is a free geographic database and world map maintained by an international community of volunteers. The very active associated Discourse community incudes over 40,000 mappers.
This Discourse community includes over 11,000 people who love to study, photograph, and discuss all aspects of the natural world.