How do multi database updates work

Hi everyone, so a simple question, how do multi database updates work in the background.

Example, I have a query that gives 10 results for example, each repeated row has 3 fields in it.
I have a multi database update form surrounding the whole thing, and after I have made changes to any of the 30 fields presented and hit update, it updates in my database perfectly.

So now my question, lets say i only update one of the 30 fields and hit update, does it still update all 30 fields but just with replaced duplicate data, or does it somehow know that I only altered one field on row 3 so it only has to update a single record.

Checking before I use this functionality on my next table that has many many more fields of data?