Without support from the vendor they don’t. Mobile computing is so locked down right now that it might be possible that not even the vendor can repair a bricked phone tbh.
Compare it to a broken BIOS on a PC. You can basically throw out your motherboard if you fail while updating it. Some devices have hardware pins used only for provisioning and debugging (JTAG) but they would have to be reverse engineered first.
Danke Karl 🕊️😶🌫️✌️