SQLite has runtime-pluggable VFS support, i.e. you give it a struct with functions for opening a file, reading some bytes, writing some bytes, synchronizing file contents, closing a file. This project provides such a VFS module, that, because it actually runs in the browser, performs HTTP requests to read data. Emscripten provides a way to run a mix of C/C++ code in the same environment as some JavaScript code inside the browser. The reason SQLite has this pluggable VFS support is to properly support embedded systems, different locking APIs, and things like database encryption.

https://www.sqlite.org/vfs.html