The answer is: it totally depends!
It depends on what information you are updating and the parameters set for your UMP which you would decide with your AgilQuest implementation team during the UMP requirements gathering sessions.
There are certain data elements that, as a standard, are updated every time the UMP runs . Things like, first name, last name, email address etc. where the source of truth is your source system (this could be Active Directory, your HRMS or other system of choice). This means that the information must be correct in your source system, which will then allow the data to flow into Forum via the UMP.
Example: A user wants their First Name to appear in Forum as Chris vs Christian.
Changing this in Forum will only be temporary if it doesn't get changed in your source system. If the feed for the UMP still contains "Christian" as the user's first name in the record, the manual change your made in Forum will get overwritten on the next UMP run.
So the data flow is from your source system (again the "source of truth"). It doesn't flow from Forum into your source system.
On the other hand, there could be some data elements on the user's profile where you have decided (again during the requirements gathering) should only be made in Forum during the initial creation of the user's record and not update every time the UMP runs. This means that a user can modify that field in their profile and it will not get changed by the UMP. (like Location or Preferred Name)
If you're not sure which data fields are updated nightly, vs which ones are only done on the initial add, refer to your UMP requirements workbook or your UMP Overview for your specific settings.