Google RSUs are taxed like your salary and wages (at ordinary tax rates). This income, and any tax withheld, is reported on your IRS Form W-2 in the year the units vest to you.
Federally, RSU income is withheld at a flat 22% until you reach $1M in stock compensation income in a calendar year, and then flips to a 37% withholding tax rate. If you expect your effective tax rate to be higher, you will owe taxes at the end of the year on this RSU income (assuming no estimated tax payments were made during the year). State withholding varies by location.