In macOS, you have the option to route all system audio through a pair of virtual channels in Console for additional level control, as well as the ability to process the signal through UAD-2 plug-ins in realtime.
Why might you want to do this? One possible use would be to EQ the soundtrack of a movie you are watching on the internet. You could also route iTunes playback through the cue outputs to be included in a headphone mix, if you were playing along to another track for example. Another benefit is that once you have routed signal through Console virtual channels, you can also easily record from those channels into your DAW software.
How do I route Mac system audio to Console virtual channels?
Open Audio MIDI Setup, which is found in the Utilities folder within your Applications folder. Once open, navigate to the Audio Devices window shown below - if the Audio Devices window isn't shown right away, you can access it by going to Menu Bar > Window > "Show Audio Devices" while the Audio MIDI Setup application is in focus.
In the left hand column, select the Universal Audio Apollo driver. Click the Output tab, then hit Configure Speakers at the the bottom right.
Audio Devices window of Audio MIDI Setup
Now that the Configure Speakers window is open, select the output channel numbers that correspond to the pair of virtual channels that you wish to route to. For most applications, choosing Stereo or 2.0 Surround will be sufficient. You can also set up multi-channel configurations for surround.
If you do not see any virtual channels available for assignment, then check the Channel DSP Pairing option in Console > Menu > View > Settings > Hardware.
Note that the channels corresponding to each output channel number will vary depending on which Apollo you are using and how you have configured the system. To see which channels correspond to which output channel numbers, check the Outputs column of the I/O Matrix panel of the Console Settings.
Now that system audio has been routed to a pair of Console virtual channels, you can begin applying UAD plug-ins to these channels, routing the signal to cue outputs, and applying whatever other creative routing you can dream up.
To record the audio from the virtual channels back into your DAW, just create a new stereo audio track in the DAW and set the inputs to correspond to the virtual channels that you have routed your system audio to (i.e., VIRTUAL 1/2 in the example above).