Why iPhones use HEIC
Apple devices save photos as HEIC because it stores the same quality in about half the space of JPEG. The downside is that many apps, websites and Windows programs can't open HEIC, so converting to JPG makes your photos work everywhere.
How Aimager converts HEIC
The HEIC decoder is a WebAssembly build of libheif that loads only when you pick a HEIC file, keeping the rest of the site fast. If your browser cannot decode a particular HEIC file, Aimager shows a clear error instead of pretending it worked.