On Mi Box, long-press the power button and volume down button on remote at the same time. Screenshots are saved inside "/sdcard/Pictures/Screenshots" folder.

On Shield TV, enable "NVIDIA Share" and long-press the home button on remote. Choose "Screenshot" on the popup window ( https://www.androidheadlines.com/2019/04/how-to-take-a-screenshot-nvidia-shield-android-tv-oreo.html ).

1. Depending on the device, if it's still under Android 8 or earlier, long-press the power button and volume down button on remote at the same time.
2. If the device is under Android 9 you can press and hold the "power" button and you will see the screenshots button.
3. You can connect a USB keyboard and press the "printscr" key regardless of the Android version or device.

The image file is saved inside a "Pictures/Screenshots" folder.