The Moodle Database Schema "reverse engineered" from the source xml files using MySQL Workbench For more about Moodle see
For more about the Moodle database structure see
Marcus Green 2017

A Mysql MWB File that can be modified. moodle_34_erd.mwb .This file contains additional diagrams that show some of the modules in more detail.
A pdf rendering moodle_34_erd.pdf. A set of images of individual components of the diagram components

How the diagram was created, Diagram FAQ. See here for an archive of  old versions. There is a file containing the xml that creates all tables here and files matching the plugin here. For development and consultancy see

Moodle 3.3 ER Diagram