[OnePlus 10 Pro] beatbreakee:
This is where your fatal mistake was.... the red line above is incorrect !

It should have been, "adb reboot bootloader" THEN "fastboot boot magisk_patched.img"

you never want to straight FLASH any modified boot image when working with an a/b partition device.. or pretty much ANY device running the dynamic partition scheme. This is because of the way the partitions are laid out. Something about flashing a modified boot.img directly breaks things... BUT you had a DOUBLE FLAW in yours because you not only flashed a modified boot img directly, you ALSO flashed "magisk_patched.img" to android, which does not have a file named "magisk_patched.img" in its manifest.... android has "boot.img" in its manifest. Understand?? I have seen this flaw ALL OVER the place and no one seems to notice that, so im hoping several ppl finally realize the MAJOR error that they are making.

The android manifest has the EXACT names of every file to be loaded into the system, and any deviations to that list must be incorporated into the BUILD structure when making the os. In other words, say you wanted to build a custom flavor of android from scratch, and have the recovery be installed as a homemade "TWRP" build. Now im talking about FULL BUILD not a port, or remake of an already built OS... I am meaning "1st install... everything from the ground, up.... similar to what lineage does" ... THAT is where the origin android manifest is built... and all updates/custom mods/roms that are made from the original that you created, MUST FOLLOW the exact same file structure, and naming scheme that is in your original manifest. recovery.img needs to be named recovery.img .... system.img needs to be named system.img .... in the case of most of these boot loop issues (BUT NOT ALL) alot of ppl either find some guide written by a person who has never rooted a device in their life, but felt inclined to serve up some untested set of steps that had a fatal flaw they overlooked. THAT is what happened here.

Whoever wrote those steps obviously never tried them before writing them, or they would have had the same result as you. You CANNOT have a completely different named boot image FLASHED to the boot partition! Those steps are instructing you to FLASH, "magisk_patched.img" to the boot partition, where android is expecting "boot.img" ... if those steps were at all to be taken to be the real intention of the author then he would have needed to add a line prior instructing the reader to "rename the file from "magisk_patched.img" and make it "boot.img" ... then in the next step it would have been "fastboot flash boot boot.img" ... but i still DO NOT recommend that, as like i said it can cause errors. If at all possible you always want to BOOT the patched boot.img file 1st, because if there is any problems with it or android rejects it, all you have to do is restart the phone, and it will boot back up to the UNMODIFIED boot.img ! ... but if you FLASH it, then it has no old boot.img to revert to... thus the boot loop..... (bad boot.img ..... reset ..... bad boot.img ..... reset ... bad boot.img ..... reset...) <-Bootloop.

When you BOOT the patched image, though and it loads into the OS with no problems, then you have confirmation that the file was patched properly and is compatible with your OS. NOW you can simply open the Magisk app, and it will see your phone as already rooted, so you can then tap the install button, and then MAGISK will install itself correctly to the right partition, AND make a backup of the file that is from your stock os.

hope that sheds some light and also more ppl take time to read this! Ill make a regular post about it shortly for more visibility.

cheers