Just like how it’s been for years and years. All of Apple’s operating systems are based on the same kernel and could easily be interchangeable between devices, it’s just the UI that’s different, and all of Apple’s chips are fabricated with the same process and could all handle the same thing. People keep thinking there’s a real difference between the OS’s and the chips but it’s all the same stuff. Apple simply puts the OS on whatever the device is and that’s it. iPhones get iOS, iPads get iPadOS, Macs get macOS, Apple Watches get watchOS, and so on and so on, it really is simple as that. Apple could 100% install macOS on an Apple Watch if they wanted to, the S chips could for sure handle it, but it would make no sense to do that.