Since iOS 11 on iPhone, the default file format for images has been HEIC. This is a new image file format that provides better compression and image quality than JPEG. On the Mac, Photos app understands and macOS HEIC since High Sierra. In the Photos app you can either use HEIC or JPEG and can also export. Other applications and devices also support HEIC.
Check out What are HEIC files? on YouTube for closed captioning and more options.