When you make your first assignment using Screencastify Submit, you will find a new folder called "Screencastify Submit" in your Google Drive. For each assignment that you create, a sub-folder with the name of that assignment will be created within that Screencastify Submit folder. Open a sub-folder to see all the videos submitted for that assignment.
You are free to move, rearrange, or rename that folder and the videos will still go into the correct subfolder for each assignment.