How does it work?

Database is supported via MySQL<->SQLite translation layer

WordPress requires MySQL. However, there isn't a WebAssembly version of MySQL you could run in the browser. WordPress Playground therefore ships PHP with the native SQLite driver and leans on SQLite.

But how can WordPress run on a different database?

Behind the scenes, the official SQLite Database Integration plugin intercepts all MySQL queries and rewrites them in SQLite dialect. The 2.0 release ships a new WordPress Playground-informed translation layer that allows WordPress on SQLite to pass 99% of the WordPress unit test suite.