https://www.cnet.com/news/you-can-now-play-youtube-music-on-your-google-home-for-free/

You can make any of those your default music service, so when you ask to play a song with a voice command, the Google Assistant built into Google Home will search your preferred service first.

The blog post announcing the update specifically mentions smart speakers, so hopefully the change rolls out to smart displays soon. You can play the free tier on smart displays if they're in a group with smart speakers, but not on their own. Starting Thursday, you'll be able to play YouTube Music for free on smart speakers in the US, UK, Australia and a handful of other countries.