The Moodle 3.3 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_33_erd.mwb .This file contains additional diagrams that show some of the modules in more detail.
A pdf rendering moodle_33_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