How to Root android 6.0 marshmallow or android 7.0 nougat or even android 8.0 oreo in very simple way.
This method applies on all mobile and also on all android versions.
First of all you need to keep in mind that these phones are not easy to root because the security level is very high and none except dovelopers can root android 6 marshmallow or android 7 nougat.
First of all you have to install TWRP recovery by any method and then using TWRP Recovery you have to install Super su file

To know how to install twrp recovery in you mobile you have to search on google like How to install TWRP recovery in your mobile name without root
Then you have to download supersu file from below and have to install in your phone using TWRP custom recovery

And by this way you can Root android marshmallow and android nougat or you can root any android narshamllow or nougat based on snapdragon or mediatek processor CPU.

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https://www.youtube.com/watch?v=iJeoQyiXIDs