As with most UI experiments, not everyone is seeing the update yet. It's possible this is currently being tested on a subset ...
The YouTube family of apps have their own design language, and the latest change is an updated icon set that is rolling out to YouTube Music.
NEW TOP 100 IN THE WORLD ICON PACK! WHICH ICON?! GUARANTEED ICON PACK + 44 RED IN FORMS IN A PACK!! CAN WE HIT 10,000 LIKES?