Thanks, @turiphro, that helps!
So since the communication protocol is common, i.e. my node uses LoRaWAN which is used both by TTN and KPN, I will be able to connect to the gateway (regardless of whether my node is TTN or KPN -- that matters next) and, given the right network key, the node will also be able to transmit its packet to the gateway and have the gateway pass it on. Is that correct? Does that mean a node is able to determine if a gateway is TTN or KPN up front as opposed to trial and error, e.g. does the gateway advertise its capabilities, similar to a wifi access point (channel, SSID, encryption, etc)?
Does that mean I could define two network profiles on my node like this: a) TTN={network key}, and b) KPN={network key} and then simply try each in turn?
Does the polyforwarder forward to one or multiple networks as in each network gets a "copy" of the packet, or is it more of a fall-back or alternative delivery mechanism (e.g. I have nodes that use KPN so I want to support both TTN and KPN nodes at the same gateway)?