[Android Q&A, Help & Troubleshooting] Mountains788:
The error that you got might be related to this one - BROM ERROR: ??(3). From this list here we can see that BROM ERROR: ?? (3) means that there is a problem with the scatter file.

About point 3.2 - try to flash your phone with the scatter file that you got from your Thunder Box backup with SP Flash Tool. If you still get the ERROR: ?? (3) message then untick BOOT and PRELOADER and see if the same message still appears. Try to select Format All + Download or Firmware Upgrade instead of the Download Only option. If that doesn't work then unselect some of the options in the white / green table in SP Flash Tool or start with only one option until you can possibly get a sucessful flash. If both your scatter file and the one that I sent you do not work, then I would try to find an alternative to SP Flash Tool to try there. You cannot just download any random firmware and scatter from the internet and expect it to work. The firmware and scatter file would need to be for a device that is like your one with similar hardware at least for it to work.

For point 4 - I have not unlocked the bootloader on my S23 clone but have done it on other MTK devices by pressing the volume+ button and it worked. I always follow this tutorial and have no problems. Perhaps it is something specific to the device? I can see that someone else has the same problem here so perhaps you can search for "fastboot unlock bootloader cannot press volume up" in Google and see if one of the solutions works for you? This could also be worth a try.

Since your bootloader is locked and you cannot enable the Enable OEM Unlocking option from the Developer Options menu, this method might not work. (and even if you did it might still not work since Volume+ doesn't work). If you have access to fastboot then you should be able to flash the firmware IMG files from your PC. Take all the files except the scatter file from the MT6735_231125052321_IMG folder and paste them into Tools/ADB_Portable/platform-tools_r34.0.5-windows/platform-tools. Open Launcher.exe and type the following:

fastboot devices

You should see some unique number and the word fastboot - 0123456789A fastboot, for example. If you see that, type in:

fastboot flash boot boot.img

If there are no errors, continue to run these commands:

fastboot flash recovery recovery.img

fastboot flash system system.img

Here is a video of someone doing this. These commands might be enough to get the phone working if flashing with fastboot is successful but you might need to flash more partitions to the device if it still doesn't boot. You can use some of the information in this tutorial or from here.

This could also be related to your problem.

You can also tell me which files you used / flashed from the internet that made the phone not able to boot. Did you use SP Flash Tool or some other method?

For point 5 - You could try and see what happens. Might have a problem with the scatter file again though.

For point 6 - TWRP lets you do flashing, backing up, wiping and other things from your device. This recovery needs to be ported to the device so that it works properly. You could try to find one that works for the same hardware in the device on the internet but the chance is very small. The ones that I have included might or might not work since they are patches with a tool of the original recovery.

For point 7 - Maybe. If you find some working custom ROM then why not though it is highly unlikely for this device since only a few people own it probably and it is a clone. You would need to find some forum for clone devices and post for help there. This could also work but I have not tried it.

Finally, if nothing works for you then try to flash the phone on another PC with the MTK drivers installed, the same or a different version of SP Flash Tool and a different cable. Maybe that could make a difference.