About HEIC to PNG conversion
PNG is a lossless format that editors and design tools accept everywhere, which makes it a good choice when you plan to edit an iPhone photo further.
HEIC photos are decoded locally in your browser, using Safari's built-in decoder or a WebAssembly build of libheif in other browsers. Nothing is uploaded.
PNG files of photos are large. If you only need to share the photo, HEIC to JPG gives a much smaller file.
Private by design
Everything happens inside your browser tab. Your image is never uploaded, so it is never stored on a server.