It’s simple. You sell a closed complement. You can’t make money with something that is free, so you can’t sustainably make money by selling open-source software. You can make money by selling something that you do not give away for free but that derives a significant part of its value from the underlying open-source software.
Examples of closed complements are closed features added to the open-source software (the now much maligned open core model), human labor in the form of consulting and implementation services, or operating the software for clients, typically in a cloud. Closed complements are additive, which is to say that you can host an open-source software in your cloud including added closed software features and customers could not be less happy.
The open-source software for which you sell a closed complement could be communally owned, or it could be owned by you exclusively if you originally developed the software and maintained 100% of the copyright. Why would you make your software available as open-source software then? This is answered by next week’s blog post about how to make more money with open source.