Hello, I haven't asked or seen when phone repair companies do flashing, but I have done it myself a lot of times. Before the explaination, I would like to warn you that you must know what you are doing since you might brick your phone. There are 2 types of bricking your phone, soft brick and hard brick. Soft brick is recoverable and usually occurs when there is a software problem like the operating system. You might go into a boot loop or the device freezes during boot. A soft bricked phone can still be recognized by the PC used to flash. Hard brick is barely recoverable and the phone is literally dead. You might have to buy a new phone if this results. A hard brick is when your phone can not boot at all. If a hard brick is recoverable at all, I recommend an expert to look at it, but most of the time it is not recoverable. To flash your phone you need ADB tools on your PC, you need to unlock your phone (by unlock I mean unlocking the bootloader not the lock screen), download TWRP, and boot to bootloader on your phone (most likely by pressing the volume-down and power button simultaneously when switching on your phone). You can then flash TWRP (follow the instructions here) and use it to flash the OS of your choice. Make sure you use an OS designed for your phone. Using OS not designed for your phone might brick your phone.