HEIC is what iPhones have shot by default since iOS 11. At the same quality it is roughly half the size of a JPG, which is brilliant while the photo stays inside Apple. The trouble starts the moment it leaves. Windows needs a paid codec just to preview it. Plenty of Android apps and older software refuse it. And almost no government, admissions, or job upload portal will take it. Convert to JPG and the photo opens and uploads everywhere, right away.
Here is the part that catches people off guard: the JPG usually comes out a bit larger than the HEIC, not smaller. That is normal, because JPG is simply the less efficient format. For anything you actually do with a photo, WhatsApp, Instagram, email, a portal upload, an ID submission, the quality difference is invisible. It only starts to matter for very large prints, A3 and up, or when you are editing a ProRaw shot, where a RAW-aware workflow belongs instead.
The friction is the same everywhere. Send an iPhone HEIC on WhatsApp as a "Photo" and WhatsApp quietly converts it to a heavily compressed JPG, so for full quality the sender has to share it as a "Document" or a cloud link, and then you convert the original yourself. Government, admission, and job portals reject HEIC outright and usually cap photos at 50 to 500KB, so the normal flow is convert to JPG first, then compress to fit.
EXIF such as the date, camera model and GPS location is not carried into the JPG, so keep the original if you need it. Live Photos convert to their still frame and the motion clip is dropped. This page converts one file at a time.
On privacy, there is no sign-up, the photo is virus-scanned with ClamAV, and files delete themselves after two hours (longer for results over 2 GB or with a Pro pass). For sensitive documents an offline tool is safer. For ordinary photos the two-hour window is reasonable. Files up to 100MB go through, which covers even big ProRaw shots. And a tip to stop the problem at the source: on the iPhone, set Settings → Camera → Formats to "Most Compatible" and new photos save as JPG.