Windows File Path
C:\Program Files (x86)\Steam\userdata\60951074\760\remote\438040\screenshots
MacOS File Path
MacOS: Making the Library Folder Visible
Command + Space opens the Spotlight-Search. If you search there for /Users/*Username*/ you will find the Library folder.
The Library folder is not visible yet? No problem:
Right click -> Show View Options -> Show Library Folder
Windows: Create Desktop Shortcut
It is tedious to look for this folder over and over, in order to have a look at his screenshot collection. My tip: Just create a shortcut for the screenshot folder on your desktop.