HEIC on Windows,
fixed.
You copy photos off an iPhone, open them on a Windows machine, and nothing happens. The files say .HEIC and Photos refuses them. Nothing is wrong with the pictures. Windows simply does not ship the decoder any more.
What HEIC actually is
HEIC is a container. Inside it the image is encoded with HEVC, also called H.265, the same codec used for 4K video. Apple switched to it in iOS 11 because it halves file size at the same visual quality, which on a phone full of photos is a large saving.
The container can also hold things a JPEG cannot: multiple images in one file, which is how Live Photos and burst shots work, plus depth maps and edit history.
Why Windows will not open it
HEVC is patent encumbered. Microsoft shipped the codec briefly, then moved it behind a paid extension in the Microsoft Store, and on most machines it is simply not installed. The file is fine. The decoder is missing.
This is also why the behaviour is inconsistent. A machine that once had the extension, or an OEM build that bundled it, will open the same file another machine refuses.
Three ways to fix it
1. Stop the iPhone making them
On the phone, Settings, Camera, Formats, choose Most Compatible. New photos are JPEG from then on. This does nothing for photos you already have, and you give up the size saving, but if you move photos to Windows regularly it is the least friction.
2. Let the transfer convert them
Settings, Photos, then under Transfer to Mac or PC choose Automatic. The phone converts to JPEG as it copies. Choosing Keep Originals sends the HEIC untouched, which is the setting most people have without knowing.
3. Convert the files you already have
Any converter will do it. The thing to check is whether it uploads: a camera roll is personal, and most free online converters send the file to a server. A converter that decodes in the page does not.
Converting HEIC to JPEG is lossy to lossy. The result is slightly softer than the original. Convert once, from the original, rather than repeatedly.
Which format to convert to
JPEG for photographs, at quality 85 or higher. It is what every program on Windows understands and the size penalty against HEIC is acceptable for a handful of images.
PNG only if you need lossless, for example a screenshot or an image you will edit further. PNG files of photographs are large, often several times the JPEG.
Do not convert to WebP or AVIF unless you know what will open them. Both are excellent on the web and poorly supported in desktop software.
What you lose
Converting flattens the container. A Live Photo becomes its still frame, the depth map goes, and any non destructive edits are baked in. If the original matters, keep the HEIC alongside the JPEG rather than replacing it.