I dont know that there is a "uniform" answer to that question. I think its more "it depends", based on the area in question, etc. I think here in california its mostly tesla employees but (for example) @jboy210 mentioned that during his solar roof install, the tesla team was training a roofing team, and more "supervising".
You might be right. I was answering based on discussion from an electrician who I had hired to do some work for me (on the Tesla "find an electrician") page. He said he used to work for Tesla and broke off to do his own thing and that when it comes to solar, only Tesla employees do it. The used to outsource panel upgrades but not anymore , he said. However, as you have stated, there is a possibility that it differs depending on region.