The positions in question here are basically lower-tier quasi-employees, who are contracted through an employment agency because Google doesn't want the liability of employing them directly. That liability is both legal and reputational, the latter because cutting contractors lets Google cut costs without worrying their FTEs. These contractors work full time for a single client (Google). They expect/hope to do so indefinitely, at least for a year or so, and in normal times they do.

I believe that UK-style contractors are closer to what I'd probably call a "consultant", specialists who get a much higher hourly rate than an FTE would but aren't sad or surprised when a contract ends. Those do exist in the USA, but that's quite different from the "contractors" here.