The wallpaper app was deleted. Its 402 MB of wallpapers were not

Animated wallpaper apps download video loops, often in 4K, and keep them so the desktop does not stream all day. Because they come from the App Store, they are sandboxed, and a sandboxed app keeps everything in its own container in ~/Library/Containers.

Why deleting the app did not help

Dragging an App Store app to the Trash, or deleting it from Launchpad, removes the app bundle. The container is a separate folder in your Library, and macOS leaves it in case you reinstall. For most apps that is a few megabytes. For an app whose whole job was downloading video, it is the downloads.

Check yours

du -sh ~/Library/Containers/* 2>/dev/null | sort -h | tail -15

# Containers whose app is not installed any more
cd ~/Library/Containers
for d in *; do
  mdfind "kMDItemCFBundleIdentifier == '$d'" | grep -q . || echo "$(du -sh "$d" | cut -f1)  $d"
done 2>/dev/null | sort -h | tail

The second command asks Spotlight whether anything installed still has each container's identifier. Treat its output as a list to look at, not a list to delete: some containers belong to helpers inside other apps.

Apple's own wallpapers

The built-in dynamic and aerial wallpapers are a different store, in ~/Library/Application Support/com.apple.wallpaper (12 MB here) plus system downloads for aerials. Those are covered in aerial screensavers and wallpapers. Containers in general are in Library Containers taking up space, and leftovers from any app in what uninstalling leaves behind.