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It is not.
It's very simple🙂
And for countries that do not have roaming agreements, it is an ideal option.
Here is my example:
I have my personal number in my phone on the physical card, which is SIM1. Calls and SMS are set up through it. I bought a card from another operator, as well as a Ubigi service card if necessary, I would add another one, for example Greece or another European country.

And depending on where I travel, I activate a card for more favorable data transfer such as SMS2 for data transfer.

It is a little incomprehensible to people who live in huge countries, like you in Australia or who lives in the USA, or Europe where there is an agreement on roaming...
But even with the agreement, by far the most favorable option is to buy a SIM (or ESIM) card locally, and still use a lot of GB for little money.
The advantage of ESIM is that it can upload several ESIMs and then choose the one that is the best option. Only one ESIM card can be active at a time.

Yeah, I just use roaming.