What is WebP?

Google's format for the web, and a genuine improvement on both of the formats it replaces: lossy like JPG or lossless like PNG, transparency either way, and usually about 30% smaller. Every current browser reads it. Software that stopped being updated around 2019 often doesn't.

What is ICNS?

Apple's icon container, the thing inside every .app bundle that makes it look like something. Like ICO it holds a set of sizes, but it goes considerably larger - Retina displays ask for 1024px icons, which Windows never did.