What is HEIC?

What an iPhone saves photos as. It isn't really an image format so much as a container holding HEVC video frames - the same codec used for film, applied to one still. That gets a photo to about half the size of the equivalent JPG, and it's also why so little else opens one: HEVC is patent-licensed, so most browsers and a lot of desktop software can't read a HEIC at all. Big photos are usually stored cut into tiles that have to be reassembled.

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.