How do I completely uninstall an app and its leftover files on a Mac?
Short answer: Click Uninstall on the app in Storage Studio's Apps window. It lists the app and its leftover files with sizes, then moves them all to the Trash so they stay recoverable.
In Storage Studio, open the Apps window and click Uninstall on the app (from the Internal or Moved tab). A confirmation screen lists the app plus the leftover files it scattered across your Mac — preferences, caches, support files, saved state and similar — with sizes and the total you'll free. Confirm, and everything goes to the Trash, so you can put it back if you change your mind.
The app is quit first if it's open. See what leftovers are removed.