Giving the core code away free and selling everything around it: hosting, support, enterprise add-ons, or a managed cloud version of the same software.
MongoDB the database is free to download and run yourself. MongoDB the company makes the majority of its revenue from Atlas, the managed cloud service that hosts MongoDB for you so your team doesn't have to. By 2024 Atlas accounted for over two-thirds of revenue, a multi-billion-dollar business built on giving the underlying technology away for over a decade.