HEIC and JPG are both lossy image formats, but they come from different eras. JPG dates to 1992 and is universally supported. HEIC arrived in 2017, backed by Apple, and uses the more efficient HEVC compression codec. At the same visual quality, HEIC files are roughly 40-50% smaller than JPGs.
Beyond file size, HEIC supports features JPG simply can't match: 16-bit colour depth, transparency, image sequences (Live Photos), and non-destructive edits stored within the file. If you're shooting on an iPhone and keeping photos in your Apple ecosystem, HEIC is the better format by every technical measure.
The catch is compatibility. JPG opens on every device, every browser, every operating system, every social media platform. HEIC doesn't. If you're sharing photos by email, posting to the web, or sending files to someone on Windows or Android, JPG is the pragmatic choice.
The verdict: keep originals in HEIC for storage efficiency and quality. Convert to JPG when you need to share outside the Apple ecosystem.