FerretDB is an open-source document database and an alternative to MongoDB. It acts as a proxy that translates MongoDB wire-protocol queries (from version 5.0) into SQL and stores the data in PostgreSQL with the DocumentDB extension. Existing applications and MongoDB drivers can therefore often connect without changes. Compatibility is still being extended, so check that your application works before going to production. You install FerretDB yourself, for example with Docker or DEB and RPM packages, or on Kubernetes with a PostgreSQL instance that has DocumentDB.
FerretDB is licensed under the Apache License 2.0. The company behind it, FerretDB Inc., is based in Wilmington, Delaware (US), according to available company information. The DocumentDB extension was originally designed by Microsoft, is MIT-licensed and is governed by the Linux Foundation. Because your data sits in your own PostgreSQL, you decide the location yourself, for example with a European PostgreSQL host or on your own servers in the EU. This limits dependence on a US document-database vendor.