Ahhhh. How bizarre. So if I'm in the iTunes store (after selecting iTunes in the left column), to search for a song I have to exit the iTunes store (which happens automatically when I press the "Search" field at the top of the left column), then select iTunes store again from the list that now appears on the upper right, THEN i can search the iTunes store.
Thanks for pointing out where the upper right iTunes option was. In the Catalina response I had looked at before starting this thread, there wasn't an image and it referred to an icon on the upper right. I had taken from that explanation that it was always supposed to be there, not only when search was selected, and was thus looking for an icon in the Music menubar instead of the text selection. Should have seen it anyway, of course, but...
Very klutzy for an Apple interface. Thanks for helping me untangle it.