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 ICO?

Windows icons, and a container rather than an image - one file holds the same icon at several sizes, so Windows can pick the right one for a title bar or a desktop. It tops out at 256x256 and supports transparency, which is what stops an icon looking like a sticker.