Step 8 – Once finished, tap on the ‘Install’ button from the list, choose the Magisk-v18.0.zip file that you copied to your phone memory in Step 3.
Step 9 – Confirm installation on the next screen by swiping on the “Swipe to Confirm Flash” option. The installation procedure should start now as show below.
That’s it! You should now have a rooted Galaxy S9 / S9 Plus device. You can go to MagiskManager app to verify.
To make sure you have correctly installed the above root method and it’s working, verify with Root Checker as given in the link.
Have fun playing with it! If you have any issues, please fire them up in the comments below!