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 fast look for quick preview of schema (*.svg file) but note NOTE that latest version and one valid valid one is in GIT repositoryView file