A:Answer To my understanding, only HDMI input 3 has the eARC feature. If you use eARC, I'd presume you would want each and all your sources to be connected to your AVR receiver's HDMI inputs, rather than the other HDMI inputs on the TV. That's how mine is wired. Hence, when using eARC to connect audio to the receiver, your PS5 and any other sources (such as a Fire TV stick or other media player, or Bluray player, or cable/satellite HDMI outputs) would each get wired to receiver HDMI inputs.
The TV can source inputs from WiFi or Ethernet ports as alternatives to HDMI3, (as when using TV internal apps) and when it does, it seemed to me audio signals to feed my receiver came out the HDMI3 eARC port, so I presume it also does that if video inputs HDMI 1/2/4 are selected on the TV. Thus, there would be no point in having more HDMI inputs with the eARC feature. When using HDMI 1/2/4 inputs, audio coming in via those inputs would come out HDMI 3 eARC to get to the receiver, amplified and out your speakers connected to your AVR receiver.
I have no idea why it's HDMI 3 that has eARC, rather than, say, HDMI 1. Probably some historical baggage.