After first appearing in April, YouTube Music is now more widely rolling out live lyrics on Android and iOS...
There are many reports of live lyrics rolling out to YouTube Music for Android (version 6.15) and iOS (6.16) today. You might want to Force stop YTM from App info before checking, and be sure to try multiple songs if you don’t see it immediately.
This rollout is not tied to the updated Now Playing UI that adds comments that’s also being tested.
The migration from Play Music wasn't great for me. There are multiple songs that for whatever reason used a random YouTube upload for the match rather than an existing official one.
Only if you listen to a video with the "audio only" setting enabled, or if you're searching for a song that isn't available in the official library, with the video being the fallback.
The actual song library is a song library, no video.