An eSIM is for calls and data. It has the same function as a physical sim. You normally get a local phone number too.
Some phones will be able to use both. Mine does. To do this, it must have dual sim capabilities, ie two sets of radios. It uses more batteries, even when idle.
Yes, if you don't have dual sim capabilities, you will need to use your Australian sim for 2FA. Check with your Australian provider if it will work (Telstra stopped all roaming for prepaid last year, causing me enormous problems, but brought it back).