What an HEIC file actually is
HEIC is a HEIF container holding HEVC-compressed stills. It stores roughly the same visible quality as JPEG in about half the space, and it can hold extras a JPEG cannot: depth maps, edit history, and the paired frames behind Live Photos.
Every iPhone since iOS 11 in 2017 shoots HEIC by default. Files reach other people through AirDrop, direct copies off the device, or an iCloud download that skips Apple's automatic conversion — which is why the problem usually appears at the moment of sharing.
What you get from JPG
Saving as JPEG re-encodes the image with lossy DCT compression at a quality level you choose. Around 80% is the usual sweet spot: files shrink dramatically and the difference is invisible at normal viewing sizes.
Nothing in mainstream computing fails to open a JPEG. Thirty-year-old software, print shops, hospital systems, government upload forms, every phone and every browser — if a system takes images at all, it takes JPG.
What changes when you convert HEIC to JPG
Both formats are lossy, so this is a re-encode rather than a copy: the HEIC is decoded to pixels and those pixels are compressed again as JPEG. Detail the original already discarded cannot come back, and the second pass removes a little more. Going to JPEG specifically means giving up the more modern codec's efficiency — expect the file to grow, sometimes considerably, for the same apparent quality. That is the price of a format everything can open.
There is no transparency to preserve — HEIC files do not carry an alpha channel, so every pixel is opaque to begin with. Converting to a format that supports transparency does not create any; it only means you could add it later in an editor.
This is the conversion most people need, and it is worth knowing what does not come with it: Live Photo motion, depth maps and Apple's edit history live in the HEIC container and are dropped. The visible photograph converts faithfully; the extras stay behind. Orientation is handled properly here, which is the other classic HEIC complaint — photos arriving sideways because a converter ignored the rotation flag.