it totally depends on the antenna vendor. the basic idea is to implement independent phase shifters along the waveguides.
sometimes, it will simply be a plastic device that will slow down the waves depending on how deep it goes into the wave guide. Oppositely, the phase shift will be removed if the device is put further away from the wave guide.
Controlling those phase shifters is done thanks to a screw at the bottom of the antenna. (the electrical tilt is mechanically adjusted :)) )