Because of the advanced computing resources necessary to properly power these features, Apple Intelligence is limited to a select number of modern devices:

iPhone 16, 16 Plus, 16 Pro, 16 Pro Max, and iPhone 15 Pro and Pro Max
iPads with M1, M2, or M4 chip
Macs with M1, M2, or M3 chip
If you buy any of the new iPhone 16 models, you’ll be set for Apple Intelligence. But if not, the only iPhones that will support these new AI features are last year’s 15 Pro models.