Hi, I'm Sergio, an Independent Advisor and Microsoft Team customer just like you, and it's a pleasure to help you.
When you access to Sharepoint,You will see there are two folders:
Submitted files and Working files. In each of these is a folder for every learner and in every learner folder is a folder per assignment.
The difference between Submitted files and Working files is that when a student uploads a document to an Assignment, it goes into the Working files folder. The learner can still open it (in Teams Assignments or in SharePoint) and make changes. Once the student has clicked on the Turn In button, it is copied to the Submitted files golder and no longer allows any changes, maybe the students don't do that.
However you can see some details about this topic in:
https://docs.microsoft.com/en-us/microsoftteams...
Greetings.