SQLite is the most widely deployed database with over one trillion active databases in use, maintained by three individuals at Hwaci. It is considered more popular than all other database engines combined, finding its origins in a US warship project. SQLite is not open source but in the public domain, with invite-only contributions allowed. Surprisingly, they use proprietary test suites and charge $120K/year for access. The unique business model includes revenue from paid support, services, and consortium memberships. SQLite is known for being extremely fast, even outperforming filesystems in some cases. It has many quirks like weak typing and limited ALTER commands. Despite its faults, backward compatibility is highly valued, with a fun fact that developers once called in the middle of the night.
https://avi.im/blag/2024/sqlite-facts/