iTunes store in Catalina

Open the Music app on the Mac, click iTunes Store in the left sidebar, select the music in the main window or use the search box at the top of the left sidebar and click iTunes Store at the top right of the main window.

Also see the iTunes Store section of Add and download music from Apple Music or buy music from the iTunes Store - Apple Support

Also, the iTunes Store option won't show in the top right corner of the main window until you click in the Search box at the top left of the sidebar.