With ARC and eARC, one channel of the HDMI output is used to send back the audio signal (e.g. Dolby Atmos), so there is no separate input for this. As far as I know, you need at least an HDMI 2.1 cable for eARC (if it says "High Speed HDMI", that's what it is).

It has 3 8k hdmi inputs, numbered 4(Game), 5(Aux1) and 6(Aux2) which can normally be e-arc. You then have to plug in the appropriate high-speed HDMI cable (from the projector to the receiver) and make the appropriate settings in the HDMI menu of the selected connection in the setup menu of the receiver (e.g. optical, HDMI through, 4k, 8k...., unfortunately I don't know exactly now). That should actually be it. I only use the optical inputs or hdmi itself and therefore cannot say exactly whether this really works. But it should work. 8k input is 8k input.
Best regards.

eARC allows two-way communication for audio, so it doesn’t matter if it’s labeled as output or input on your devices. Just connect the eARC compatible HDMI on your projector to the Denon AVR and make sure to enable eARC on both devices.

If your projector is only being used as a display, it won’t send video or audio to the AVR. Instead, the media source (like a DVD player or Nvidia Shield) sends video to the projector and audio to the AVR. Make sure the media device supports formats like Dolby Atmos or 3D sound. If your projector has built-in apps or plays media directly, eARC would let it send audio back to the AVR.