What is ICNS?
ICNS is the macOS icon container - what sits inside an .app bundle. Since Lion it stores its images as ordinary PNGs at a handful of sizes, which is why reading one means picking the biggest member and writing one means rendering several.
What is ICO?
ICO is the Windows icon container, and still what a browser fetches when it asks a site for /favicon.ico. One file can hold several sizes at once so the system can pick the right one; each dimension is stored in a single byte, which caps an icon at 256 pixels.