I used to sync media on my computers (2 Mac, one PC - all running iTunes 12.4) by using Home Sharing to connect from my current computer to one of my others, and filtering the view to show media not in my local library. From there, there was a button to import those "missing" media files.
This facility (both the filter and the import option) no longer exists. Or has it moved? I can see my other library, and play media from it, but not import any. It was there in 12.3!
