Like many things Apple, I do not find this to be actually true in practice outside the apple ecosystem. What I find is that some of the photos are .jpg and some are .heic and the live photos are .MOV videos when pulled from the device.
So the apple devices do not capture and store all their images in .heic / .heif when set to do so. But they do store some. I did not realize they were set this way even until I went pulling data copies when we started the phone changing process.
Anyways, I cannot view any of the images that I have pulled that are .heic extension with gthumb. Also same thing with image viewer/eye of gnome. It makes me think I am missing some support package maybe since I am still in 20.04 LTS
Haven’t tried too much because I had to install 20.04 last night, but early findings…. Following the steps given, opening .heic files is sporadic. Like if I browse the device directly, gthumb works fine. If I copy the files to the pc, gthumb is very hit-or-miss as to whether it opens them. Some it opens fine, most are broken images. Installing the package @fossfreedom mentioned made no difference, and installing libheif mentioned somewhere else also had no effect.
On 22.04, .heic images work correctly out of the box. Only issue is if you edit an image using gthumb, it can’t be saved as .heic. But gthumb and Nemo thumbnails work.