Most certainly not, which means that they'd be cached in local storage.
Though local storage of video might become a pain for offline viewing.
I will also have to purchase more local storage to perform my backups.
To make this short, local storage should only be considered as a last resort, or for very specific deployments.
MS doesn't care about local storage for home users.
All of the songs are streamed, so there's no need to rely on local storage in order to listen.
With no local storage, the water continues flowing into Mexico.
This leaves very few cases in which a terabyte of local storage is a necessity.
In other words, if you don't need local storage for a phone, why even use it?
So they will back up your own local network-attached storage to the cloud.