What is HEIC?
HEIC is what an iPhone saves photos as, and it is not really an image format so much as a container holding HEVC video frames - the same codec used for film, applied to one still picture. It stores a photo at roughly half the size of the equivalent JPEG, which is why Apple switched to it, and it is also why so little else opens it: HEVC is patent-licensed, so most browsers and a good deal of desktop software cannot read a HEIC at all. Large photos are usually stored cut into tiles that have to be reassembled, not as one picture.
What is BMP?
BMP is the Windows bitmap: pixel values written out with essentially no compression, which makes the files enormous and the format trivial to read. It survives in Windows tooling, embedded systems and anywhere a program wants image data without a decoder in front of it. Rows are stored bottom-up.