We expect the iPhone 15 Pro and 15 Pro Max to be equipped with the 3nm (nanometer) A17 Bionic chip, while the rest of the iPhone 15 range (iPhone 15, iPhone 15 Plus) will feature the A16 Bionic manufactured by TSMC.
The reason not all phones will have the newer processor is the high cost of silicon production – according to Phone Arena Apple's Pro phones will be the only ones all year with a 3nm process.