How To Open System Directories In Finder Without Opening A New Finder Tab
You can enter those directories from keyboard with ⌘ CMD+↓ Down arrow.
 Or ⇧ SHIFT+⌘ CMD+G and type the name of the directory You want to enter. You can complete directory name with ⇥ TAB button (for example type Down in $USER dir, press ⇥ TAB and it will become Downloads).