Hi there iwaddo,
Thank you for contributing to the Apple Support Communities.
I see you're looking for an easy way to switch between audio sources when using your Beats Studio Wireless headphones.
The main way to accomplish this is to first stop using the headphones as your iMac's output device (in System Preferences) and then use the AirPlay icon in Control Center to use the headphones with your iPhone. You can find these steps in the following articles:
OS X El Capitan: Set up Bluetooth audio devices
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Choose Apple menu > System Preferences, then click Sound.
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If your device is meant for sound output (for example, headphones), click Output. If your device is meant for sound input (for example, a headset), click Input.
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Select the device you want to use in the list of devices.
Set up and use third-party Bluetooth accessories
If you want to play audio to a Bluetooth accessory, you'll need to select the accessory.
You can do this by using the AirPlay option in Control Center.
All the best.