So, iPhone 15 and 15 Plus and everyone else gets most of the software features available in iOS 18 except for Apple Intelligence, which due to 8GB RAM requirement and faster Neural Engine in A17 Pro limits access to iPhone 15 Pro and 15 Pro Max until iPhone 16 Pro and 16 Pro Max gets released...
So, iPad 6 with A10 is limited to iOS 17 while iPad 7 with A10 can get iOS 18
iPhone 5s [A7] and iPhone 6 [A8] are both limited to iOS 12
iPhone 6s [A9] and iPhone 7 [A10] are both limited to iOS 15
In the future, iPhone 13 series may get cut off earlier than iPhone 14 and 14 Plus even though they all use the same A15 Bionic, and iPhone 14 Pro and 14 Pro Max may get cut off earlier than iPhone 15 and 15 Plus even though they all use A16 Bionic
Buying the repackaged iPhone 14 Pro and 14 Pro Max [iPhone 15 and 15 Plus] seems to be a very not so wise decision if you are planning to use the Apple Intelligence features offered in iOS 18... This is the earliest that newly released [2023] models are immediately obsolete...