How do these FET's work back to back?
As far as I understood, FET's are not meant to be used bidirectional.

They sure can operate bidirectionally but, with one proviso; in the reverse direction, you cannot get the MOSFET to fully turn-off (because of the bulk diode) but, in all other respects, the MOSFET will turn-on to a low value \$R_{DS}\$ when the current is in either direction providing that the gate-source voltage (for N channels) is sufficiently positive: -

Picture above from here.