Blackview BV4800 MT6761V

For Rooting a mediatek we need some tools

1. MTKclient : MTKclient on GitHub

2. Magisk : Magisk on GitHub

MTK client get boot file

install mtkclient on computer

connect device to your computer and connect it to mtklcient

download boot.img with MTKclient

python mtk r boot,vbmeta boot.img,vbmeta.img

or

python mtk r boot_a,vbmeta_a boot.img,vbmeta.img

For some another device  the file who need patch its "recovery.img" or "init_boot.img"

Install Magisk and patch the boot.img

Copy the file Magisk-v27.0.apk and boot.img to the smartphone storage using the adb push command. Then install the apk file (you must grant the install unknown apps permission to the Android file manager).

adb push boot.img /sdcard/Download

Launch the Magisk app on the phone and select Magisk ⇒ Install ⇒ Select and Patch a File: point the program to the boot.img file that you uploaded into the phone storage. After a little of work you should obtain a modified boot image saved into a file like /sdcard/Download/magisk_patched-27000_xxxxx.img. The image should contain a modified ramdisk, which will provide the su command (superuser) when the Android system is running, but actually without modifying the system partition.

Download the patched boot.img to your PC using the adb pull command.

adb pull /sdcard/Download/magisk_patched-27000_xxxxx.img ./

signed file 

https://github.com/chenxiaolong/avbroot