Hi,
ERP5 uses ZODB as its main database. It is slowly transitionning to our homebrewed NEO storage, which is a distributed database following the same API and implementing the same features as ZODB.
Also, https://mariadb.org/ is used as a catalog of ZODB/NEO, indexing the most commons and shared properties of all ERP5 documents and their fulltext representation, in order to easily and fastly query the ZODB/NEO storage.
Regards,
Nicolas