In fact Siri was originally an app (Apple bought it). To your question, a simple example is "Hey Siri" https://machinelearning.apple.com/research/hey-siri http

In fact Siri was originally an app (Apple bought it). To your question, a simple example is "Hey Siri" https://machinelearning.apple.com/research/hey-siri https://machinelearning.apple.com/research/hey-siri > To avoid running the main processor all day just to listen for the trigger phrase, the iPhone’s Always On Processor (AOP) (a small, low-power auxiliary processor, that is, the embedded Motion Coprocessor) has access to the microphone signal (on 6S and later). We use a small proportion of the AOP’s limited processing power to run a detector with a small version of the acoustic model (DNN). When the score exceeds a threshold the motion coprocessor wakes up the main processor, which analyzes the signal using a larger DNN. Yes, as with all things they could come up with a way to let third-party apps program the AOP with a different detector, but my point is that Siri has deeper integration into the device. Apple's privacy and security rules also limit what a third-party app could do. For example, Siri knows what apps you have installed, but apps aren't able to access this information. Siri can make modifications to system settings; apps cannot.