The clue is in the names of the drives C: is for the OS (Operating System) which in your case is Windows 10. All new programmes will be downloaded to the C: drive unless you specifically change it. The D: drive is used for data i.e. Documents, Pictures, Video, Music etc.
By splitting the hard drive into two partitions (you do actually have a single hard drive of 1.5Tb) you have the possibility to update, refresh etc the Operating System without making any changes on the drive containing the data. So if for any reason the OS crashes due to a bad update to Windows or one of the other programmes on the C: partition you can repair it, replace it without affecting any of your data.
Keep them separate, do not move the files on the C: Drive to the D: drive unless there are data files on there.
I am surprised that no partition is shown for restore to factory settings, but that may be hidden.