Why is the RAM usage of my system smaller on startup than it is after using (and closing) software?
It is not something you want "fixed", as this is intended behavior.

It is "costly" (in terms of time) to load new data into RAM, as is done for example when opening applications such as Photoshop and InDesign.

However, it doesn't cost anything to keep the data in RAM, and there's no process of "erasing" stuff from RAM before you load new stuff in.

Therefore it makes sense to keep for example Photoshop in RAM after you close it - in case you want to open it again soon. In that case, there would be parts of Photoshop already in RAM that do not need to be loaded again - saving you time.

If you go ahead and open up other stuff that need more RAM, those Photoshop parts would eventually be overwritten with new stuff.