Open an iPhone photograph on a machine that refuses it.
This file stays on your device
Drop files here
Up to 30.0 MB
Lower makes a smaller file.
An iPhone has saved photographs as HEIC since iOS 11, and almost nobody chose that setting. The trouble turns up somewhere else: a Windows desktop, an Android handset, a print shop counter, an insurance claim form, or an older upload box that answers with "unsupported file". JPG is the one every single one of those takes without argument. This is the conversion that makes a camera roll usable away from the phone that filled it.
The JPG is usually the larger file, sometimes twice the size, because HEIC compresses far better than a format from 1992. Whatever the HEIC carried besides the picture is dropped as well: the depth map that portrait mode records, and the short piece of video that makes a Live Photo move. The still picture is what survives.
Keep the original in the camera roll. This street runs one way. Writing a HEIC needs an HEVC encoder covered by patents, so nothing here can turn the JPG back into one. Convert a copy.