The Apple TV's "TV Speakers" setting is a bit of a misnomer. It refers to sending audio out from the HDMI port. Other speakers that may appear in the menus are available via Airplay 2 or bluetooth. Setting it to "TV Speakers" is correct for audio to be sent via HDMI to the receiver. This info can change to "Receiver Speakers" depending on the receiver used and if HDMI-CEC is enabled. If the designated source input, in this case the Apple TV 4K, displays on the front of the Onkyo, then that is where the audio is coming from during playback. If it was in an ARC loop, the TV input would be displayed on the front of the Onkyo as it switched from the directly connected device, the Apple TV 4K here, to the ARC input. Make sure the Audio Format settings in the Apple TV 4K are set correctly. So, for Audio Output, it should be set to TV Speakers and the Audio Format should be set to Auto with Atmos Available. Oh, and hello and welcome to the forums.