You write exempt in one of the boxes. They’ve hidden the part where it tells you which one. You can look it up, but its not stated clearly on the form anymore.
But you still have to pay taxes if you owe them, and you have to pay a penalty if they didn’t withhold enough. But if you’re poor enough you don’t owe federal income taxes you can avoid giving the IRS an interest free loan if you file your form properly.
The US learned what happens if taxes are actually voluntary during the days of the articles of confederation. Results were predictable.