On my desktop PC even when I shut down all programs a third of the available memory (8GB) is in use. I know there are always some programs and processes that run anyway but according to what I can see using the Task Manager that doesn't account for more than a fraction of that number.

Can anyone please explain to me what I can do to find out what's using all this memory or if this is just normal when running Win 10 Pro?

I don't have an exact explanation. But I just checked in the Task Manager the Details tab and looked at the available columns. There are lots of memory values i.e. about peak memory. Maybe that is what is used for the overview summary. This screenshot shows only a small part of the list on my PC.

Yep, just checked, it's lying it's little teeth off.

 

If you change the display to percentage it seems to be closer to the truth (right-click on the header, resource values, memory, percent)

Also the resource monitor is a lot better.

 

I also don't have an explanation, but on my computer it's the same, so i guess this is pretty normal.