That narrative is really not true. Tesla provides the resources for repairs and parts just like other U.S. auto manufacturers.
Anything that deals with the high-voltage (HV) systems need to be certified for obvious safety and liability reasons.
Tesla Parts Catalog
https://epc.tesla.com/en
Service and Diagnostic website
https://service.tesla.com/
You can access "Service and Repair Information" and even "Diagnostic Software" and pay by the hour, day, month or year...
Some manufacturers won't even provide some of these to individuals and only makes them available to dealerships.
My friend and I just performed maintenance on our Model 3s. We changed the A/C filter and cleaned the condenser and serviced the brakes. I just ordered the parts from the Tesla app and did everything in my garage.