Music on iPhone not showing up in iTunes on macOS

If I connect my iPhone to my Mac via USB cable, iTunes launches and shows the iPhone icon in the toolbar. But if I click that, no content whatsoever is displayed under the "On My Device" header (except one iBook I bought). Does anyone know how to solve this? I would like to manage the music downloaded to my iPhone via macOS iTunes, as removing downloaded music on the iPhone itself is a disaster in user friendliness. I have iTunes Match, Apple Music and running macOS Mojave 10.14.5 with iOS 12.3.1.