https://docs.moodle.org/dev/Tutorial needs a section that explains:

• autoloaded locations - https://docs.moodle.org/dev/Automatic_class_loading
• callbacks (absolutely magic functions in lib.php's that might seem like redundant code for new devs) - https://docs.moodle.org/dev/Callbacks
• core components
• plugin types locations
• internal and non-internal php files

For new devs moodle file structure may be confusing