Connect wireless accessories
Using Bluetooth® technology, your iMac can wirelessly connect with devices such as the Magic Keyboard, Magic Mouse, Magic Trackpad, AirPods, peripheral devices, wearable sport accessories, and more.
Before you connect a Bluetooth device with your Mac for the first time, you need to pair them. When a Bluetooth device is paired with your Mac, you can connect or disconnect it using the Bluetooth 
menu.
Some included accessories may already be paired with your iMac. For more information about accessories, see What’s included.
Connect a Bluetooth device. Turn on the device so that it’s discoverable, then open System Settings and click Bluetooth. Find the device you wish to use in the list, then click Connect next to it. The device remains connected until you remove it. Control-click a device name to remove it.
Turn Bluetooth on or off. Click the Control Center icon 
in the menu bar, click the Bluetooth icon , then click the control to turn Bluetooth on or off. Your iMac comes with Bluetooth turned on.
Tip: You can add the Bluetooth icon to the menu bar. Open System Settings, click Control Center, then use the pop-up menu next to Bluetooth to select “Show in Menu Bar.”
Learn more. See Use Bluetooth devices with Mac in the macOS User Guide.