Okay, I work at a fulltime job so please bear with me and sorry for my late responses. And acknowledge that I am not responsible for anything that happens to your device. This step-by-step guide was done on an unlocked Google Pixel 3 XL unlocked bought from the Google Play store. I haven't tested it out on other versions of Pixel 3's or any other phone that isn't the Pixel 3 models since I do not own them, so keep that in mind. My situation was to find out how to update my ALREADY ROOTED/MAGISK version of Android 9 PIE through OTA. Upon checking or looking around I couldn't find any tutorials that are easy for me so with all the info I gathered I decided to wing it and compile steps from a lot of sites,risk it all and used my knowledge to put together this guide. In hopes, this helps everyone else! anyways here we go...
First, PLEASE make sure if you have Edxposed or any other modules or anything that is similiar, uninstall it or remove it. (Applications or mods from magisk are fine, I did this while I had a few mods from magisk, like viperfx, youtube vanice etc..)
1. Disable Automatic system updates in developer options, so it won't install OTAs without your acknowledgment.
2. When an OTA is available, first go to Magisk Manager → Uninstall → Restore Images. Do not reboot or you will have Magisk uninstalled. This will restore your boot back to 100% untouched stock images in order to pass pre-OTA block verifications.
3. After restoring the stock boot image, apply the OTA update as you normally would (Settings → System → System Update).
Wait for the installation to be fully done then press the "Restart now"
Wait for the phone to reboot and let it sit for a few minutes. 10-15 minutes give or take.
Don't panic, let it do its thing.

Once you see the "G" with the loading under it, you should be fine.
let it boot up.
4. Now download the factory zip
download
10.0.0 (QP1A.190711.020, Sep 2019)
5. Once downloaded, open the zip using any zip/rar program. (I HIGHLY recommend WinRar) within THAT zip folder extract the image-crosshatch-qp1a.190711.020.zip somewhere on your desktop.
6. Now then open THAT zip file and from it, extract the boot.img file onto your desktop and move it into your Pixel 3 XL's download folder. (I did it this way, so I recommend it)
7. Now uninstall your old magisk manager and install the latest Magisk Manager (app-release.apk)from
HERE
8. Open Magisk Manager and Press Install ==> Press Install again ==> Select Patch a File ==> Select the boot.img file you extracted from the latest factory image that you placed in your Pixel 3 XL.
9. Magisk Manager will patch the boot.img and store the patched boot.img in the download folder as "magisk_patched.img"
10. Copy the magisk_patched.img file to your computer's Platform-tools folder(The one you use to adb push files in etc.) or the folder that has fastboot.exe(your ADB folder should have the fastboot.exe in it normally lol

) in it.
11. Connect the phone to the Computer with USB type- c cord.
12. Open your platform-tools folder if you haven't, and on the search bar type in "cmd" and tap enter on your keyboard.
(the cmd promt should open already in your location)
13. Now boot your Pixel 3 XL into fastboot mode by typing the commad: adb reboot bootloader
14. Once your phone is on fastboot mode then type: fastboot flash boot magisk_patched.img
15. Once it's done then type the command : fastboot reboot
16. Google logo should show,but don't panic give it a few.
Open Magisk and check if its rooted on the manager
And congrats ! you have a Magisk root on Android Q ! :highfive:
Please let me know if it worked for you and let me know if you need any help! :fingers-crossed: