It depends on the plan you choose. Our single-country Australia plans currently connect to Optus. Some multi-country plans that include Australia use Telstra instead.
Every plan card on our Australia eSIM page lists the exact network that plan connects to, so check it before you buy if a specific operator matters to you. After you land, your phone joins the plan's network automatically; you do not need to choose it by hand. If you see no signal, make sure data roaming is turned on for the eSIM line.