My employer pays all my income (I don’t have any side gigs). Unless I do something unusual like take a few months off for parental leave or similar, they know I make 12x my monthly salary exactly so they deduct my taxes from the table of income tax for someone having my yearly pay. It’s going to be almost exactly correct.

Reasons for deviations on the final tax would be non-income taxes/deductions (selling stock, income or deductions from interest).