What a HEIC File Is
HEIC is the format iPhones and iPads have saved photos in by default since iOS 11. Technically it is a HEIF container carrying an image compressed with HEVC, the same compression family used for modern video. The engineering is genuinely good: a HEIC photo is roughly half the size of an equivalent JPG at the same visual quality, which is why Apple switched, phone storage fills half as fast.
The format can also hold more than one picture in a single file. Burst shots, live photo frames, and edit history can all live inside one HEIC, and it supports transparency and wide color as well.
The catch has never been quality, it is compatibility. HEVC compression is covered by patent licensing, and much of the software world declined to pay. The result is a format that is excellent inside the Apple ecosystem and a frequent roadblock outside it.