Why would the working people necessarily need to be taxed? You could pay for UBI with taxes on investment, or wealth, or luxury taxes, or other things besides labor.
Working people in general. Those who keep producing and not sit on their asses enjoying UBI. You can tax incomes, wealth, whatever. Either way it’d be taxing those who want more than UBI. And you’ll need to tax them a lot.
There’s no way taxing super-rich-only would cover fair UBI. It’ll have to be much much wider. Unless 99% of jobs would become automatized.