how does one change the format from HEIC to JPEG or something compatible for Etsy?

Personally I prefer to have my iPhone (also applies to an iPad) to save the pictures as .jpg instead of .heic for more universal support. There are two settings within iOS depending on exactly what you want. One setting keeps the pictures on the iPhone in .heic format, but converts them to .jpg when exporting them to macOS (and possibly Windows - does not work for Linux). The other setting has the pictures saved as .jpg on the iPhone.

On the iPhone or iPad launch the "Settings" app and scroll down to "Camera". Within the Camera settings you can change check: "Keep Normal Photo". The other setting is found within "Formats" where you need to check "Most Compatible".

I'm not at my Mac at the moment, but I have used a third party app to convert an .heic picture to .jpg, but I don't recall the app at the moment. I'll see if I have it on my Mac tomorrow.