YouTube Signature Device is a tag given to the phone which passes criteria required to achieve the best-in-class video experience YouTube has to offer. That is: HDR, high frame rate, 4K decoding, and next generation video codecs support.
Check out the list here.
Due to the YouTube policy changes, Montage video with focused on violence will demonetize. Violence in the normal course of video gameplay is generally acceptable for advertising, but montages where gratuitous violence is the focal point is not, such as “The Best Kills in Resident Evil 7” or “The Best Kills in Call of Duty: WWII”.