Heic?

HEIC is a newer standard for digital images than JPG. Its images have higher resolution than JPG and are smaller than the equivalent JPG image. Most photo editing software that has been updated in the past couple of years supports HEIC now. When you export images from an iPhone it will convert them to JPG if the destination does not support HEIC. But if you want all images taken with the phone to be JPG go to Settings/Camera/Formats and change the setting to Most Compatible.

If you have pictures that are already existing and want to convert them from .heic or .heif to JPEG:

I can explain what I did with pictures from my iPhone which were also in the "photos" app on my Mac. I selected the pictures I wanted to convert, and then used the function "File -> Export". Now a window appears where you can choose the format in which you want to export the pictures, so I took JPEG and exported them to the hard disk on my Mac. That's all.