The best open source alternative to MongoDB is PostgreSQL, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to MongoDB and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to MongoDB are SQLite (Free, Open Source), MySQL Community Edition (Free, Open Source), MariaDB (Free, Open Source) and Redis (Free, Open Source).
Open Sqlite databases and explore the internal organisation of various objects such as schema, tables and indices. To understand how Sqlite3 data is organised, please see [Database file format](https://www.sqlite.org/fileformat.html) from Sqlite website.