Does Windows 10 use 1 or 2 partitions for system files and user data?
By default, no. It does not. All your user data is stored under your user profile at
But it is trivial to move your
My Documents and other folders to another drive, and most programs easily allow you to change install locations.
Users directory or profile directories under it is tricky though.
But this is the same as Linux, Linux can use a single partition and end up with programs and user data on the same drive as well, it all depends on how you set up the machine to begin with and how configurable the installer is.
Not out of the box, but you can set it that way, though it requires a bit of gymnastics. You can set paths for your user folder on a different partition if you like (your Documents, Downloads, etc.); see How to Move Users Folder to another Location in Windows 10
As I said, it takes some preparation and a little expertise.