The Roxi service leverages ATSC 3.0 and internet connectivity to provide on demand music videos. I would guess the “xx.21” video subchannels are a pointer channel that launches an internet app, or the .21 is just a base video “home” with the internet doing most of the work.
sbgi.net
roxi.tv
ROXi, Revolutionary Interactive Music TV Channel, Launches on US Broadcast TV - Sinclair, Inc
ROXi and Sinclair partner to bring interactive music TV channel to NextGen TV households in 31 US markets, with music from Universal Music Group, Sony Music Entertainment, Warner Music Group, and MERLIN LONDON & BALTIMORE – ROXi, the world’s first fully interactive TV music channel, powered by...
sbgi.net
ROXi - Free Music Channel
ROXi – Enjoy music videos, karaoke and games on your TV. Get the ROXi TV music app free on Sky Q, Amazon Fire TV, Android TV and Google TV.