How can users enable Siri on their Apple devices?

To enable Siri on Apple devices, users can hold down the Home button or simply say, "Hey Siri." This method is designed to provide quick and intuitive access to the voice assistant without the need to navigate through menus or download additional applications. The ability to activate Siri with a voice command is particularly useful for hands-free operation, allowing users to perform tasks, ask questions, and control their device while keeping their hands free for other activities.

The other methods mentioned are not valid for enabling Siri. Tapping the screen twice does not invoke Siri and is not a recognized gesture for this purpose. Downloading a separate app for Siri is unnecessary since Siri is a built-in feature of Apple's operating system and does not require installation from the App Store. Turning off and on the device might help in troubleshooting but does not specifically activate Siri as a feature. Therefore, the correct answer emphasizes the most efficient and recognized ways to get Siri to respond on Apple devices.