Firmware replace or update throught serial (UART) port with USB TTL

The most devices running mini servers can be update via serial port, just like routers or IP cameras. As far as I know, the firmware version of this smart lock can also be updated/modified via USB TTL.

Is this firmware “modification” process deeper than the OTA (over the internet) update process? Does it overwrite all files and directories? Does it actually re-flash the memory of the device?

During a traditional update (OTA) process, is it possible that the software is only partially upgraded?

I don’t know how public this information, but what system is the device running? Is it custom developed or some linux based, linux distribution?

The first time (after paired the phone with the lock) I try to connect to the Internet, even if I enter the password correctly, it won’t connect. I have to go into the settings about wifi again to enter the password and then it connects. I have updated the system, I don’t know what wrong.

Is it possible that I need to completely “flash” or reinstall the system?

When a new update is released, does it overwrite the existing API or the whole system?