iPhones don't even have QHD (2560 x 1440 px) screen resolution yet! My iPhone 13 Pro Max screen is only 2778 x 1284. That is only slightly better than full HD at 1920 x 1080, certainly not 4K resolution, and no where near 8K resolution. You can definitely view 4K and 8K video streams on the iPhone 13 Pro Max, but you will only see them in less than QHD resolution. Perhaps you are confusing the camera resolution for screen resolution. They are NOT the same thing at all.
As far as upgrading the ATV 4K to an 8K model goes, don't hold your breath. Until 8K TV becomes the standard "new TV" resolution, Apple won't mess with 8K. They will wait until there are enough 8K TVs in homes to make it worth their wild to upgrade the ATV. Since 4k is just now becoming the norm, don't expect an ATV 8K for at least a few years, if ever. To stream 8K people will need a lot more bandwidth, so that has to become available as well.