Help with Photos on iPhone/MacBook Pro - uploading to photo-storing sites

Look, I am SERIOUSLY Mac-challenged. I still have trouble with Finder because I'm so used to the Windows OS. I've had this MacBook Pro since 2015 and don't even have a clue as to how to organize my photos. (I'm slowly learning.)

But I am VERY clueless when it comes to how the iPhone photos are stored on the Mac. What is an "HEIC" extension???

When I try to upload a photo from my iphone/Mac to a website like Etsy, all I see are my photos with an "HEIC" extension instead of "JPG" and the HEICs are NOT "uploadable". Is this an iPhone setting?

I also have this problem with Shutterfly and Snapfish. I'm trying to get print-outs of these photos, but can't upload them to the sites because all of the photos have this ". HEIC" extension.

Is there a way to save the photo with another extension (?) so that they are available to upload?

I've tried to save as .jpg and have had NO LUCK.

Thank you, Apple Community.

Simone