The 4k60 ProRes mode is not available for shooting on the 128gb model until an external drive is added, but for any larger capacity iPhone Pro, the mode is available for shooting without the external drive. This doesn't affect the Pro Max as that isn't available in a 128gb configuration.
Notably, a similar limitation existed with the iPhone 14 Pro using 4k30FPS, at the time the reasoning was that it simply fills the device too quickly to be useful.
You are 100% correct and my original data is wrong. (imore.com)
Apple’s website lists the external drive requirement for 4k60FPS:
4K at 60 frames per second (fps), iPhone 15 Pro and iPhone 15 Pro Max only, when using an external storage device that supports speeds of at least 220MB per second and maximum power draw of 4.5W
Unfortunately there is no way to edit my above comment, so my apologies for being the source of incorrect information here.
The 4k60 ProRes mode is not available for shooting on the 128gb model until an external drive is added, but for any larger capacity iPhone Pro, the mode is available for shooting without the external drive. This doesn't affect the Pro Max as that isn't available in a 128gb configuration.
Notably, a similar limitation existed with the iPhone 14 Pro using 4k30FPS, at the time the reasoning was that it simply fills the device too quickly to be useful.