[Xiaomi Mi A2 / 6X Guides, News, & Discussion] :
Hey hey hey !!! I'm back for a new method, for flash Magisk without patched_boot.img or select current-slot etc, just by do 2 things. And it works for all phone version, Sable, Beta.... ( We use TWRP temporarily, so after the process, TWRP is gone)

Credits :

Me for the idea.

@Frustrated Indian the tester.

@TingyiChen for this nice TWRP.

Your warranty is now void.

We are not responsible for bricked devices, bootloop... Please
do some research if you have any concerns about features
included in this GUIDE before using it! YOU are choosing to
make these modifications.

Of course first you must have the drivers installed on your pc. And an unlocked bootloader + unlock critical partition, if it is not, follow this procedure :

1. Installing drivers & Preparing PC

- Download this and extract it

- Open and install "adb-setup-1.4.3.exe"

- When it is done, go to "Drivers" folder and start "miflash_unlock.exe"

- Click on the gear icon top right corner and click on "Check" next to "Click the button to install the drive"

- Do as the installer says

- Hopefully the drivers has been installed so we're done with it, let's head on the next step.

2. Preparing phone for unlocking

- On your phone navigate to Settings->System->About phone and tap the build number until you see "You are developer"

- Go back once and you'll see a new menu called "Developer options"

- Tap on it and search an option called "OEM Unlocking", enable it

- We're done here. Let's keep going

3. Entering fastboot and unlocking bootloader (MAKE A BACKUP of your data!)

- Hold your power button down, tap on restart and hold your volume DOWN button while it resets until you see the Mi bunny doing its thing

- Now we're in Fastboot mode, hooray

- Download this zip and extract it on your PC

- Open it and type "cmd" (without the quotations) in the file manager header where you see (This computer>somethingsomething>Mi A2 fastboot), don't be afraid just type it in, and hit enter and you'll enter into command prompt

- Now we are ready for action

- Type these commands one by one:

fastboot flashing unlock

fastboot flashing unlock_critical

- And follow phone on screen steps to validate unlock bootloader...

After this, you will see a warning message that your bootloader is unlocked and your data is not safe, blah blah... Just ignore it everytime you power on your phone.

Downloads :

- TWRP here

- Magisk Latest Beta / Stable (if you prefer) zip

- Magisk Uninstaller zip

Steps :

- Download TWRP

- Copy it to your pc adb folder

- Rename it to twrp.img

- Download Magisk Installer

- Download Magisk Uninstall

- Place these 2 folders on your phone storage

- Reboot phone to fastboot

- Open fastboot command prompt on your pc

- Type "fastboot boot twrp.img" without quotes

- Your phone will boot twrp

- Decrypt data with your pin if needed

- Click install and select the Magisk Installer zip, swipe to confirm

- Wait for the process

- Reboot

That's it !! You now have root on your phone, and TWRP is gone.

If you have issues you can uninstall Magisk with the Uninstaller zip.

For OTA just uninstall Magisk using Magisk Manager, by flash the stock boot.img to your current-slot, or using the zip and flash it like the Magisk Installer.

Check my other thread, Part 3 : OTA for more informations.