Probably mentioned elsewhere, but; There are currently 3 Networks in Oz - Telstra, Optus and Vodafone.
Other than the 3 main networks (above) all others providers are known as Mobile Virtual Network Operators (MVNO)
All MVNOs buy space on one of these 3 i.e. Belong, Aldi, Boost, Dodo etc etc etc. Some only buy space on the Wholesale network in the main capital cities, others also buy space in bigger regional area i.e. Bendigo, Wollongong etc. I know Aldi works in Maryborough VIC. But the moment you venture away from these areas you have no connection even if you register a signal.
As I mentioned Boost operate on the Telstra network, the big difference is that they operate on the "entire" network. Wherever you get Telstra you get Boost and to my knowledge the only MVNO that does so. Virgin operated on the entire Optus network, (I used to use Virgin) but was largely bought out by Optus. I believe Sir Richard retained 5% and the name, but a few years ago it was absorbed into the Optus brand and discontinued.
Many phones locked to the Telstra Network will work on Boost without needing to be unlocked. (I think Boost have a list of phones that should work) As I mentioned, the Flip 3 (not a smart phone, but internet capable) is the only 4G handset I've found that has an external antenna port. My previous T96 also had one, but it is 3G only and that part of the system is gradually being shut down by all 3 Networks with the introduction of 5G.
I recently escaped VIC before the last border closure. I had 5 weeks on a mates property, no Optus signal of any kind, needed me to get within 5kms of Inglewood to get one. With the Flip 3, no signal anywhere on the 10 acres, but I put up an old bullbar phone antenna on top of a telescopic pole at about 5 M high and fed it into the caravan. I had phone and full 4G internet the whole time. The external antenna made a huge difference.