When you deposit to Swyftx, we automatically convert your AUD deposits into USD via foreign exchange conversion. This happens instantly and automatically for your and the inverse happens when you withdraw AUD.
Because we credit your AUD deposits with USD, we can show you the AUD value of your balance by using the current foreign exchange rate as an estimate. This rate changes through out the day as the global markets move which is why you see your AUD balance changing also. Your actual account balance of USD never changes, and remains the same quantity regardless of any AUD/USD market movements.
If you're comfortable trading in USD or BTC, then you may want to change the base asset dropdown to one of those, which will no longer display your USD balance in AUD.
I just sold another asset for AUD, and my AUD balance is changing?
When trading against AUD, you're using your USD balance, whether you're buying or selling. The AUD values shown throughout the platform all represent the value of the USD, not AUD itself.
What this means is that when you sold your asset, you swapped it for USD. Your USD balance won't change without you depositing/trading/withdrawing, but the AUD Value of your USD changes with foreign exchange markets.