Hello Anton
Your question is similar to another thread with the subject "Moodle architecture diagram" which has a few useful links, but not to the E-R diagram you're looking for, as am I. The DB diagrams generated by MySQL Workbench are better than nothing, but they're too detailed for those of us trying to get an overview of the Moodle system and then 'drill down' to see more detail. To use an English-language saying, in the DB diagram one can't see the wood for the trees.
Fred