This is why I specified for 90% of people. Most people don't have significant taxable income beyond employment (their only other income being savings account interest which is handled by your bank by a similar but separate system), so their tax liability is their income tax on their employment.

A lot of people in the software industry specifically might have significant investments, shares, maybe rental income, etc, and then you do need to specifically inform the government about those, but this isn't the case for the 90%.