On my computer, my devices show up in the “Devices” section of the Music app’s sidebar, but when I select a device, I only see the list of tracks present on the device.
I assume you can use this interface to manually manage your music, by dragging tracked to and from the device.
But if you want anything more robust, you must click the “Sync Settings…” button in Music, which causes the corresponding Finder window to open.
The TV app is similar. You can see what’s on your device (and I think, manually manage video content), but sync’ing is primarily done via the Finder window.
Podcasts, Books and Photos have even less. Podcasts and Books have no way to see anything on your device. And Photos only lets you see the device’s camera roll (so you can import the images). In order to transfer anything from your Mac to the device must be through the Finder’s sync window.