Every platform with Apple Intelligence has an Apple Silicon chip (A- or M-series) chip, and at least 8 GB of RAM. The iPhone 15, 15 Pro Max, 16- & 17-series phones, and iPhone Air all have 8 GB of RAM. Part of the idea behind Apple Intelligence is to do a lot of on-device processing for privacy and security reasons – but that means using RAM for large on-device models.
The iPhone 15 and 15 Plus have 6 GB of RAM. So they don't get Apple Intelligence.
How to get Apple Intelligence - Apple Support
Some Apple Watches can access Apple Intelligence features when they are paired with Apple-Intelligence-enabled iPhones nearby. The Watches do not have nearly enough RAM to run Apple Intelligence themselves – and they are presumably using the paired iPhones to do the heavy lifting.