What about managing music that one has to manually add through iTunes, such as songs purchased through Amazon Music? Is there an easier way now in iOS 10 instead of having to download from Amazon on my PC, then drag and drop onto my phone using iTunes?
It's possible, but convoluted. You can either use that method, or you can use iCloud Music Library
Full how to:
1) Import songs to iTunes
2) Enable iCloud Music Library in iTunes (under Preferences)
3) Allow your library to upload and match
4) Enable iCloud Music Library on your iOS device (Settings > Music > iCloud Music Library)
5) Open Music
6) Tap LIbrary
7) Find upload by Artist, Albums, etc.
8) Tap Add icon
9) Tap Download icon