iOS has a daemon that reads your notifications and ships them to Apple Watch. They have a daemon that scans for AirPods and gives you UI to pair them. But you as an app developer cannot do any of those things. There was no public API for notification stream access, scanning for specific Bluetooth devices, floating UI widgets, or even just persistent daemons. All of those capabilities more or less exist on Android, which is why multiple smartwatch ecosystems have been built on top of it while iOS only supports the first-party option.
Back in the 2000s, when Apple was just getting into mobile devices, the app development landscape was far less bleak. iTunes on Windows could happily index your entire music and video collection and sync it to an iPod and there was nothing Microsoft could do to stop them. Everything is just finding the appropriate file and connecting to the appropriate USB device to transfer it. And that's more or less how things still work today, except now on smartphones all of that is put into isolated containers and walled off behind private APIs.