Open the Android app now and you'll be greeted by a white splash screen with a Play Music logo that morphs into a YouTube Music one.
What Google didn't think of is an easy way to manage your locally-stored data on your phone.
If you've downloaded music for offline listening, you can't do that anywhere within the app now, and that data will still take up space on your phone.
There, you should be able to clear the GPM's cache and offline music.
RIP, Google Play Music (2011-2020)