These can be created by the students or teachers themselves or use existing online software.
Example
Joomla! is open-source software which can be used to build a community.
In the classroom
Whether teachers create a new community or use an existing commercial site, there are numerous features that can be exploited in the classroom, including forums, bulletin boards, chat, personal profiles, and galleries.
Further links:
https://www.teachingenglish.org.uk/blogs/nelson-arditto/online-learning-communities
https://www.teachingenglish.org.uk/article/using-web-resources-create-tasks-real-outcomes