Hopefully to clear things up.
The Port will receive power from an electrical outlet. Your amp receives power from an electrical outlet. Your passive speakers receive power from the amp.
The Port will be an audio source for your 6 channel amplifier. If your amplifier can use multiple audio sources, then you can connect multiple Ports to it. How the amplifier uses those audio sources is up to your amplifier. Most I’ve seen will allow you to send different audio sources to different zones/rooms, but don’t know your specific amplifier.
Volume control is as I stated earlier. You can have it set so the Port(s) use a standard flat volume, no control on the Sonos end, or you can enable variable volume control through the Sonos app. Your choice, whatever works best for your scenario.