They are provisioned remotely over network from controller run by ISP, which sets up appropriate timeslots (xPON is a time-division multiplexed network in practice, though DWDM - wavelength modulation - is also used).
Internet -> ISP network -> PPPoE -> OLT (head unit) -> passive fiber network (and/or DWDM) -> passive splitter close to home (often in inside appartment buildings) -> ONT -> ethernet to your router for PPPoE -> your systems