[Upgrading, Modifying and Unlocking] goofnorf101:
I have a Blackview A20 Pro (note this is probably different in specification to the A20) - Here is how I rooted it by using Magisk on a boot image read and written using SPFlash tools. I had previously unlocked the bootloader on the phone by using the Developer option for OEM unlock and then fastboot unlocking as standard.

- Obtain the MT6739_Android_Scatter file from an archive of the a20 stock rom available by googling

- Install Magisk manager apk on the phone

- Run the latest version of SP Flash tools on the PC with the associated drivers. Load the scatter file in to it.

- Work out the size of the boot partition by subtracting the start value of the next partition (Logo) from the start position of (Boot)

- I did using Windows calculator in programmer mode - set to hex, subtract the two values. Note the start position of the boot partition

- Go to the Read Back page, add an item to read back, enter a filename to save it like a20-stock-boot-image.img and then enter the start position of the boot partition and the size you calculated. Make sure to get them exactly correct and avoid changing the number of digits in the entry box as I don't know how fussy sp-flash tools is.

- Unplug the phone from PC if it is plugged in and remove the battery.

- Tell SP Flash tools to start the read-back

- Plug the phone in, within a few seconds it should read and save the boot partition from the phone. Keep this backed up. File should be something between 8-25mb I would expect.

- Put the battery in the phone and power it up and unplug/plug it in to the PC via usb and then in the notification area on the phone select "Transfer Files" from the very bottom so it appears to your PC as a drive.

- Drag over the stock boot image from your PC somewhere on the phone - like the Downloads folder.

- On the phone open magisk and select install - when prompted select to use local boot image, select the stock boot image file you transferred

- Magisk should now patch this file - make sure the text it outputs looks like it recognized the boot image and is working.

- Now unplug and replug the phone to PC as I found PC doesn't seem to see new files on the phone. Pull over the patched boot image Magisk has made. Don't worry if this is smaller than the original file. The extra bytes were likely just zeros.

- Using SP Flash tool, go back to the Download screen and select the patched boot image for the boot partition. This will write the new boot image to your phone.

- Power down the phone and remove the battery.

- Tell SP flash tool to start and then plug the phone in with no battery. It should then flash the patched boot image.

- Put the battery back in and boot the phone. When you open magisk app on the phone now it should say you are rooted.

Enjoy!

If it doesn't work, repeat the download process with the stock boot image you saved and that should restore things to how they were before. I assume if it doesn't work it will just be because you need to unlock the bootloader or because your phone is different to the scatter file, but in that case I would expect Magisk to fail to patch it before you get to that stage.