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Drop one HEIC or two hundred. Batch conversion is not a paid feature here.
An iPhone photo landed on a Windows machine, in an old CMS, or in a web form that only takes JPEG, and nothing will open it.
HEIC stores the picture as an HEVC-coded still inside a HEIF container. JPEG re-encodes it as baseline DCT, which every piece of software on earth can read. Shooting date and GPS survive in EXIF; the file usually gets larger for the same visible quality.
HEIC holds 10-bit colour and JPEG holds 8, so the wide-gamut highlights in a bright sky flatten out. If the source was a Live Photo, only the still half converts, because the video half is a separate file on the phone.
HEIC has been the iPhone default since iOS 11, but HEVC still image decoding is licensed patchily on the desktop. Windows needs a paid codec pack for it; JPEG needs nothing.
At a high quality setting the difference is invisible on screen. HEIC is simply a more efficient way to store the same picture, so the JPEG ends up two to three times larger.
Settings, Camera, Formats, Most Compatible. New photos are shot as JPEG from then on. Existing HEIC files stay HEIC.
Drop one HEIC or two hundred. Batch conversion is not a paid feature here.
JPEG Image is lossy, so the quality control decides the trade between size and fidelity.
Converted and deleted the moment it finishes. Never written to permanent storage.