You don’t have a hard and fast threashold but have it taper off on a progressive curve.
Either way, both require little extra bureaucracy.
For what it is worth, from an economic perspective, the two ideas are basically identical. In a very hypothetically scenario: If someone gets a UBI of $500 a month, and then makes $500 in income with a 20% tax, than they would net $900 for the month. With an NIT, you would design it so that they get a tax credit of $500 per month, and then every $500 in income reduces their tax credit by 20%. So that same person would get $900 of the month.