What changes when you convert HEIC to JPG
HEIC is what an iPhone writes by default. It is HEVC video compression applied to a single frame, wrapped in an ISO base media container, and it is roughly twice as efficient as JPEG at the same visual quality. That efficiency is why Apple chose it and why almost nothing else can open it: HEVC is patent-encumbered, so browsers outside Apple's do not ship a still decoder for it.
The conversion here loads a WebAssembly build of libheif when you add a file — not on page load, because most visitors are reading rather than converting. The frame is decoded to raw RGBA, drawn to a canvas at full resolution, and written out as JPEG at a high quality setting. Rotation is applied during the decode rather than left as an EXIF flag, so the result appears the right way up in software that ignores the flag.
Expect the JPG to be larger than the HEIC it came from, often by about double. You are converting for compatibility, not for size, and the increase is the price of a format that everything can read. If size is the actual problem, compress the JPG afterwards rather than accepting a low-quality conversion.
Two things do not survive. A Live Photo's motion clip is discarded, because a JPEG can hold one frame and nothing else. On portrait shots the depth map goes too, which means the background blur that was already applied stays baked in and can no longer be adjusted. Recent iPhones also write an HDR gain map that JPEG has nowhere to store, so a photograph of a bright sky can look slightly flatter than it did in the Photos app.
If you convert HEIC often, the better fix is upstream: set Camera to 'Most Compatible' on the iPhone and the problem stops occurring. On a Mac, Preview exports to JPEG with the colour profile handled properly. This page exists for the machines where neither of those is available — a work laptop without administrator rights, a shared computer, a phone that is not yours.
This page is the HEIC-to-JPG case specifically. The general tool, with its full set of limitations and questions, is HEIC to JPG.