Cat M1 and Cat NB-IOT have different capabilities.
Optus and Vodafone are rolling out Cat NB-IOT networks with Cat M1 due out later.
Telstra is doing a dual Cat NB-IOT and Cat M1 rollout
NB-IOT do not automatically reconnect - that is one of the weaknesses so you specifically have to tell the device to reconnect and find a tower
In terms of who’s towers (Optus, Vodafone or Telstra) will be used depends on what the roaming agreement that Hologram has with Australian Providers.
For 4G if you configure the APN to be hologram most often I am getting Telstra towers (I am not sure if this is a roaming agreement or something else)