In the UK you get a code based on last year’s earnings, which the company uses to set a flat rate of withholding on each paycheck. If there’s any discrepancy that usually just feeds into next year’s code.

In Australia, you probably need to tell the companies about the other income sources, and they will attempt to withhold at the appropriate rate. Then at the end of financial year, you go to your pre-filled online tax return which has all the figures reported by each company you work for already present and sums up whether there’s a refund or payment due. This is also where you enter any deductions.