Difference between Root Volume and User Volume?

The root volume is where the OS and applications normally live. The user volume is meant for your users to store their data in. In the case of a rebuild of the WorkSpace the root volume gets reverted back to default whereas the users data is persistent, at least as much as its last snapshot, normally every 12 hours.

In your case you'd want to store your video content on the user volume or possibly even some external shared storage like FSx or EFS depending on the type of WorkSpaces you are deploying.