macOS df reports wrong volume for disk cleanup

On modern APFS macOS, df -h / shows the sealed read-only System volume which always looks nearly empty (e.g. 16GB used). Real user data lives on /System/Volumes/Data — you must run df -h and pick that line (or just inspect ~/Library subdirs directly) to see actual capacity pressure. Big invisible hogs include ~/Library/Containers/com.docker.docker (VM disk image, tens of GB), ~/Library/Application Support/com.apple.wallpaper (video wallpaper cache, often 4GB+), and ~/.Trash which never auto-empties.