i see that one of my hdmi ports has (ARC).
Do you mean, pc -> tv -> monitor?
ARC is the "audio return channel". So you connect the source to the TV and use ARC to connect the AVR for audio. But ARC has limited bandwidth, equivalent to an S/PDIF connection - so it cannot do "HD" audio formats.
eARC is "enhanced ARC" - a new feature which finally supports a full-bandwidth return channel and should work with all HDMI audio formats. This is how HDMI audio should have been from the very beginning in my opinion.
However some specific implementations of eARC may not support all formats yet. I believe LG's implementation does not support Dolby Atmos for devices connected via HDMI - though it's possible they might via a firmware update, as it does support Atmos via the built-in apps.
Depending on what you're trying to achieve with your setup, and what your budget is, there are many different options.
If the only issue you're trying to solve is connecting the AVR to the PC without affecting audio or video, I believe the best option would be to use a passive (
not active) DisplayPort to HDMI adapter to connect the AVR via HDMI - which I completely forgot about earlier.
That way you have the HDMI 2.0 port on the PC connecting directly to the TV for features such as 120Hz and VRR support, and a separate cable going to the AVR for audio that is completely independent of the TV.
A more expensive option, but one which should work with all devices, is to replace the AVR with one capable of eARC (or use a device which converts eARC to a regular HDMI output).
Then you would hook up your source devices to the TV, and connect the AVR via eARC. This will let you send all video formats to the TV, and use most audio formats via the AVR (because LG does not currently support Atmos via eARC).
In my previous post I suggested another alternative - an HDMI audio extractor. This takes one HDMI input and splits it to two HDMI outputs: video+audio for the TV, and an audio-only output for the AVR.
The extractor I suggested should support most video formats and all audio formats, including Atmos, and does not require the AVR to support eARC.
But I've now realized that while it supports the video formats that most sources will use, it probably does not support 1440p120 or HDMI-VRR - which you'd want if you are connecting a PC to the OLED.