Where is developer documentation?

Do you have any developer documentation? I am trying to add data to the bottom of the customer page to view extra info, but I have to work out the database structure and write custom SQL queries.

Normally plugins have developer docs we can call on their functions.

It’s also all written in Javascript, so I have to write my functions in AJAX code and fetch the user ID from the hash value in the URL to pass to my PHP script.

It all seems very closed and difficult to extend which is concerning. So any developer documentation to help?

Thanks