Database used by eCloud is Cassandra. This is NoSQL database so there aren't typical relational schema.
The schema created using 'drawio' free software (https://drawio-app.com/)
is in GIT repository here: https://git.man.poznan.pl/stash/projects/EC/repos/europeana-cloud/browse/docs
Below is quick preview of schema (*.svg file) but NOTE that latest version and valid one is in GIT repository!