The most common image file type amongst smartphones is indeed the Joint Photographic Experts Group file type, or JPEG for short. JPEGs have proven to be of high image quality, universal and relatively small in size and no company has decided to change that in recent years besides Apple and the iPhone.
Similarly to Apple's reasoning for opting to use AAC compression in their audio file types as opposed to MP3, since the iOS 11 update Apple has decided to use HEIC compressed images in comparison to the more traditional JPEG images for a few reasons with the main one being that HEIC images take up less space on the device while still providing great image quality.
HEIC images also have more advantages as well that include improved highlight quality, improved detail in the shadow parts of the image, improved midtone ranges, a greater dynamic range as well as 16-bit colour whereas JPEGs only offer 8-bit colour profiles.
Take a look at the video below by the YouTube channel: Apple Explained on Why iPhones Take HEIC Photos...