Different TVs and their different HDMI-CEC implementations will change the display of certain devices concerning the HDMI connection. The eARC feature was supposed to free folks from having to use HDMI-CEC to get audio back to an AVP/AVR. But, certain TVs will turn HDMI-CEC ON automatically to use the eARC feature BUT it does NOT need to be ON at the receiving end to use eARC. If using ARC, not eARC, HDMI-CEC must be ON at both ends.
When using a newer Onkyo Receiver with HDMI-CEC ON in the receiver and ON in every device, the Onkyo remote controller can be used to control devices that allow it over HDMI-CEC. So, if using HDMI-CEC and selecting the Apple TV 4K as the source device, simply press the MODE button on the Onkyo remote controller and CEC will display on the front panel. It will now control the Apple TV 4K. To tweak receiver settings, simply press the MODE button again to return to receiver control. RCV will display on the front panel to signal the return to the default remote control function.