What is PPM?

Uncompressed RGB behind a four-line text header - about as simple as an image format can be while still being one. Nobody ships PPM to users; it exists as the thing scripts and teaching code write when they'd rather not link an image library.

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.