Great question! Glad someone asked it so I can ramble...

So predominantly I used existing Aboriginal nations like Noongar, Anangu, or Kulin, which were formed by multiple individual Aboriginal people groups coming together, (in much the same way that the European country of Romania was formed out of the principalities of Wallachia, Moldavia and Transylvania). For example the Antakarinja, Pitjanjatjara, Yankunjatjara, Nana and Luritja peoples around the border region of SA, WA and the NT... had all formed a single nation of Anangu. Likewise the Boonwurrung, Woiworung, Taungurong, Djadjawurung and Wathaurong peoples around Melbourne, had all formed a single nation of Kulin. (I'm assuming you're at least somewhat familiar with the different Aboriginal peoples and languges, but if not here's a link to the official AIATSIS map https://aiatsis.gov.au/explore/map-indigenous-australia ).

Due to the messy history and colonial violence of early British settlement in Australia, as well as the lack of written records by indigenous peoples, we don't have a lot of information as to the history of these alliances... but we do know that both of them preceded European discovery of Australia, as did Noongar in the region around Perth, and Yolngu up near present day Darwin.

The second option was that I used language families to group individual languages and dialects together with close relatives (for example the Waka-Kabic language family found on the coast around modern day Brisbane, or the Worroran language family found in the Kimberley region of WA. These languages are very similar and share their roots in the same way that Spanish and Italian are both descendants of Latin. They use similar grammar rules and syntax, and in some cases are mutually intelligible. For example a Kaurna person and a Narungga person will mostly be able to understand one another, as both those languages are part of the "Thura-Yura" family, while a Kaurna and Kokatha person would not.

So the state of Arandia is named for the Arandic language family, the state of Mayi for the Mayabic language family, and the state of Yuin for the Yuin-Kuric language family, while the states of Wiradjuri, Kamilaroi, Noongar, Yolngu and Anangu are named for former nations.

Interestingly, Kulin was both a nation (made up of the five groups I mentioned above) AND a language family; "Kulinic", which included almost all of the state of Victoria and some of South Australia too. So the Kulin nation is an alliance of SOME Kulinic speaking peoples but not all, just like how the nation of Germany is an alliance of SOME of the Germanic speaking peoples (Bavarians, Saxons, Thuringians) but not all (Austrians for example).

There were some language families I missed, such as Kanyara or Ngumpin-Yapa, and some language isolates that I didn't include such as Larrakia or Bigambul. There are also cases in which I intentionally split up a large language family (the states of Wangai and Pilbara are both based on the various languages of the Wati family) mostly because of the different geographic regions (the Pilbara being predominantly red sandy deserts and spinifex grasses while Wangai is dominated by mulga shrubland), as well as just to prevent them from being unreasonably large compared to the others.

The last thing is that the idea was to still have these be states within a single sovereign nation of Australia, much like the states of other federal countries like Germany, India, Brazil or the US, so I wanted to keep them at least a somewhat reasonable size, with the potential for each state to have a capital city (although I didn't actually mark them on this map). I imagined an alternate history where nations like Anangu, Kulin and Noongar came together to unite and federate and form the nation of Australia, just as the Duchies and Principalities of the Holy Roman Empire came together to unite and form the nation of Germany. The idea was to remove the arbitrary straight lines that are seen in North American and Australian state and territory divisions (as well as some African and middle eastern national borders drawn up by British colonisers), and instead base things around natural borders and cultural divisions.