How can CPUs operate at such a low voltage with billions of transistors?
The less important part of the answer is that in order to go fast and consume less power, CMOS chips use transistors designed to operate at very low voltages (by 1990 standards).
The more important part is that you are thinking of transistor arranged in series, like this:



simulate this circuit &ndash; Schematic created using CircuitLab
But in a modern CMOS chip, the transistors are in parallel -- actually series-parallel, but only short series strings.  Kind of like this.
It's common to have three or four in series, and some in parallel, but not more than that:



simulate this circuit
Search on &quot;CMOS logic circuit&quot; for more information.