Question. Imagine that I have a row in my data with a field e.g age = 35 and I perform an update mutation which sets age = 35 (same value) then updatedAt is not changed? Correct?

yes

But updatedAt is not updated when I add a new file relation. I have not tested with other types of relations though

that is also correct

Ok. then I have to add an additional date field to check if relations have been changed right?

yes.