Samsung Internet lets you play videos in the background
This isn’t a One UI 6 feature per se, since you can get it on other devices with the latest Samsung Internet browser installed. Nonetheless, it’s mentioned as part of the One UI 6 changelog, and it is easier to spot on Samsung phones that come with the Internet app preloaded.
With background play enabled, you can practically play any video in the background. I’m not subscribed to YouTube Premium or YouTube Music Premium (I’m more of a Spotify person), but I like to listen to the occasional playlist of fan covers. Samsung Internet makes it possible for me to run a playlist on YouTube (accessed from the browser and not through the YouTube app, of course) and push that tab to the background without interrupting playback.