Which part of the "Back up all data" do you not understand. That is made perfectly clear before you send anything in. So if you lose data due to something needing a repair. Then tough luck that's on you.
When you spill soda on your laptop so it won't turn on anymore, good luck making a backup.
Once upon a time, you could just move the SSD to a replacement board, but Apple decided to solder down the chips so now board replacements have to destroy customer data. And the repair program doesn't allow the shop to do anything other than replace the board, so there's no possible way for an authorized shop to preserve data.
Meanwhile, the unauthorized shops can just replace a few blown chips, clean the corrosion off the board and return the computer fully functional, for less money and in less time.
I doubt most consumer brands would officially support component level repairs of a PCB. But a few years ago I found the schematic and PCB component placement files for my old 2008 MBP so that info is out there.
Yes. These documents are available and independent repair shops use them all the time. Apple will say that possession of them is illegal and will sue people who redistribute them. That's not helping independent repair. Its actively punishing any shop that wants to do more than act as a proxy for Apple's mail-in service.
If Apple is serious about supporting independent repair shops, then they should be making these schematics and board-view documents available. If they are worried about copyright and industrial espionage, they can distribute them under an NDA, signed as a term of joining the program.
But Apple is not serious. They are scared that the growing right-to-repair movement is going to end up creating laws that will force Apple to make documentation and parts available under reasonable terms. They don't want that to happen so they're creating programs like this, that don't actually do anything, but seem to look nice to someone who doesn't actually do repairs (like politicians).