How do LoRa end-devices configure their upstream and downstream communication?
I'm not sure about AU, but in the US, typically the gateway will listen on a set of 8 consecutive upstream channels called a sub-band. The end device will try transmitting join requests on random channels across all sub-bands until it gets a response. The device will then frequency-hop within that channel's sub-band, changing frequency after every packet.

Join responses, like all downstream packets, are sent on one of the downstream channels. Usually it's channel associated with the upstream sub-band, i.e. downstream channel 0 goes with the first 8 upstream channels, downstream channel 1 goes with the next 8 upstream channels, and so on.

Your end device's transceiver will need to transmit on the upstream channels and receive on the downstream channels. LoRaWAN inverts the signal polarity for downstream channels, so the end devices normally don't receive each other's packets.