Most of my important files are in Google drive, music are in Google music. You upload them once and those files stays there every time you change your phone. For files in Google drive, you can open it with the Google drive app, and if you want to you can download them locally onto the phone, but a copy of it stays in Google drive in the cloud. Same with music, you stream it when you want to listen to a song, but you can also download them locally. A copy will always stay in the cloud. I personally don't see the need of always needing 20 thousands songs on my phone all the time.