Mobile Virtual Network Operators, or MVNOs, are telco companies that do not own network infrastructure (mobile towers, etc) but instead lease these services from an operator and resell these services to its customers.
There are only 3 mobile networks in Australia: the Telstra, Optus and Vodafone networks.
Every other service provider is an MVNO. Each will indicate which network powers its service. For example, Amaysim uses the Optus 4G network while Aldi Mobile uses the Telstra network.
MVNO customers get the same access to these networks as customers of network operators. No one is given priority or preferential network access.
The key advantage of choosing an MVNO supplier tends to be price. These are smaller companies with lower overheads and the savings are passed on to the subscribers.
For more information about which MVNO uses which network read: MVNOs Explained: Which Network is Behind Your Carrier?