Core developer of an open source software suite here. We make money by doing the following things:
We offer a free and open source community package that has the basics. However, we offer a number of professional packages that we offer yearly subscriptions for.
We host our software. We charge by the number of active users.
Custom development.
Paid support like migrations or troubleshooting. Also helping external developers develop custom modules.
This allows us enough income to develop the community part of the software.