Nvidia Shield
For Nvidia Shield (2017 darcy) running Android 9.1.1
If you are currently in bootloop, download the recovery image
Extract/flash using official instructions
Basically...
fastboot flash staging blob
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img
fastboot flash vendor vendor.img
fastboot reboot
Make sure you are on Android version 9.1.1; You may need to update via OTA on the Shield itself if Nvidia doesn't have the most up-to-date package on it's website.
Download Magisk 25.2 apk from official repository:
On computer...
adb install Magisk-v25.2.apk
adb reboot bootloader
fastboot flash boot magisk_9.1.1.img
fastboot reboot
You should now be able to use Magisk.
To restore/unroot
adb reboot bootloader
fastboot flash boot stock-boot_9.1.1.img
fastboot reboot