Where do I find the Junk Yard tab?
Short answer: The Junk Yard tab sits in the top row of Storage Studio, right after Apps (Home | Apps | Junk Yard). If it's missing, turn on Enable Junk Yard on this Mac in Settings → Junk Yard.
It's in the row of tabs at the top of Storage Studio's main view, right after Apps — the row reads Home | Apps | Junk Yard. Click it to see the full board.
If the tab isn't there, the Junk Yard is switched off on this Mac: turn it on in Settings → Junk Yard → Enable Junk Yard on this Mac. You can also skip the tab altogether and press the open shortcut (⌃⇧J by default) to bring the board up in its own window.