The answer to that is depend on the manufacturer...Whether that's Tesla, SolarEdge or Enphase or LG or whatever, I assume Tesla still needs parts from someone else if it's not their part.
Tesla isn't even listed as approved contractor in CA from what Vines posted anymore:
www.cpuc.ca.gov
I had an issue myself a while back and called my installer, they told me to wait 24 hours or something. I called Enphase and the guy on the phone literally fixed my problem while I was on the phone (not some dispatcher, they actually fix stuff).
Wanted to add Tesla solar isn't even always cheaper sometimes. I posted quotes/examples in my case and it was maybe $0.05 cheaper? My PTO was 1 week.