Why Windows cannot open your iPhone photos
HEIC is a container around HEVC-compressed images — the same codec used for H.265 video. HEVC carries patent licensing that Apple pays for and Microsoft historically did not bundle, which is why a photo that opens instantly on a Mac or iPhone shows as an unreadable file on a Windows machine, and why the fix Microsoft offers is a paid codec from the Store.
That licensing question is why this conversion exists at all. Technically HEIC is the better format: roughly half the size of a JPEG at comparable quality, with support for 10-bit colour and depth maps. Practically, JPG is the one that works everywhere.
What comes across, and what does not
The image converts at full resolution. What does not survive is everything HEIC can hold that JPEG cannot: the depth map behind Portrait mode, the extra frames of a Live Photo — you get the still — and any HDR gain map, so a photo shot in HDR may look flatter than it did on the phone.
The conversion runs on your own device, which matters more here than for most formats: these are usually personal photographs, and most alternative sites ask you to upload them. If the resulting JPGs are larger than you want, run them through image compression, and JPG to PDF will bundle a batch into one document.