[Nvidia Shield TV & Shield TV Pro (2019) Guides, Ne] Krijos:
Based on the work done in https://xdaforums.com/t/how-to-root-and-install-magisk-on-nstv-2019-pro-v8-2-0.4189567/

User friendly guide written for Windows. Bash script for Linux flashing edited as well, but guide is focused on Windows users.

Enhanced upscaling is confirmed to work, have not been able to confirm AI upscaling (Unsupported content). Dolby Vision unconfirmed as I do not yet have a TV that supports it. If someone can verify if this works or not I will update this post accordingly.

Prerequisites:

USB-A male to USB-A male cable.

USB mouse.

USB drivers: https://developer.nvidia.com/gameworksdownload#?search=SHIELD Family Windows USB

Android SDK: https://developer.android.com/studio/releases/platform-tools

Premade archive: https://drive.google.com/file/d/1XDzQyWviwVbjxNRsWJ8pG-HPSKW-Kzyl/view?usp=sharing

Unlocking the bootloader:

Go into Settings > Device Preferences > About.

Select "Build" several times until "You are now a developer!".

Go into Settings > Device Preferences > Developer options.

Activate USB debugging.

Connect one end of your USB-A to USB-A cable into the port furthest away from the HDMI on your SHIELD TV and the other into your computer.

Open a command shell (command prompt, powershell, bash, etc) on your computer.

Navigate to where you extracted the Android SDK.

Run command:

adb devices

Confirm that your device appears in the list of devices. If not this may be due to:

USB cable not connected properly. Check cable.

Device not recognized. Reinstall drivers/restart computer. Generic Android USB drivers may work if NVIDIA doesn't.

Run command:

adb reboot bootloader

Wait for fastboot to start.

Run command:

fastboot devices

Confirm that your device appears in the list of devices. If not see above reasons.

Run command:

fastboot oem unlock

Flashing:

On your computer navigate to where you have extracted the archive from the post.

Run command:

flash-all.bat

If all goes well you should be rebooted into TWRP recovery once the procedure is done.

Connect your USB mouse to the USB port closest to the HDMI port on your SHIELD TV.

Slide to unlock.

Go into Mount.

Select "Enable MTP".

Move "Magisk-v23.0.zip" from the downloaded archive to your SHIELD TV.

Select "Disable MTP".

Go into Install.

Select "Magisk-v23.0.zip" and install it.

Go into Reboot.

Select "System".

Getting Magiskhide to work:

Install Termux from Google Play.

Open Termux and run:

su

cat >/data/post-fs-data.d/bootfix-post.sh

resetprop ro.boot.verifiedbootstate orange

Press Ctrl+d.

Run:

chmod +x /data/post-fs-data.d/bootfix-post.sh

cat >/data/service.d/bootfix-service.sh

resetprop ro.boot.verifiedbootstate green

Press Ctrl+d.

Run:

chmod +x /data/service.d/bootfix-service.sh

Open Magisk (Settings > Apps > See all apps > Magisk > Open)

Select the shield icon on the bottom bar.

Enable MagiskHide

Reboot system

ALL DONE!

Sources of the premade archive contents are the following:

Android SDK: https://developer.android.com/studio/releases/platform-tools (fastboot)

The following Shield ROMs from https://developer.nvidia.com/gameworksdownload#?search=2019&tx=$additional,shield:

NVIDIA SHIELD ANDROID TV 2019 Pro Developer Only OS Image 8.1.0 (vbmeta_skip.img)

NVIDIA SHIELD ANDROID TV 2019 Pro Developer Only OS Image 8.2.0 (mdarcy.dtb.img)

NVIDIA SHIELD ANDROID TV 2019 Pro Developer Only OS Image 8.2.3

Latest TWRP recovery from https://twrp.me/nvidia/nvidiashieldandroidtv2019.html (recovery.img)

Latest Magisk from https://github.com/topjohnwu/Magisk/releases