FETs have a neat party trick: they can conduct in both directions. And, a bias on gate-to-drain will turn on the FET as well as the more customary gate-to-source.

In this circuit then, the left-hand FET will be on, bringing up the source for both: the two sources will be at the same voltage as the left-hand FET drain. So the right hand FET will also be on.

This slide deck gives more information about how FET biasing works. tl, dr: it’s about gate to *substrate* bias. https://alan.ece.gatech.edu/ECE3040/Lectures/Lecture24-MOS%20Transistors.pdf

One detail: the IC brings the gate voltage *above* both FET drain and source voltages since n-FETs are being used. This ensures that turn-on will occur, since Vgs is higher than threshold.