User Felthry has said already many important facts. This transformer is designed for flyback DC to DC converters. The winding inductances, core size and the used materials are designed to fit certain voltages and 50kHz switching frequency. The center tap in the primary gives a possiblity to more complex than basic applications for competent engineers who can do the needed calculations without application notes.
AUX is for the regulation circuit.It needs power , too. There's a kickstart circuit for start, but long time consumption needs a supply. This way the circuit feeds itself. Feedback from the output is generally made optically to retain the isolation.
To use this you should learn how flyback converters work and are designed. Converter IC producers have good application notes. The general principle is to charge some energy from input DC to the primary winding (=to its magnetic field) and when the current has grown high enough, the primary current is turned off. The resulted inductive spike around the magnetic core is directed through diodes to capacitors in the outputs. It appears in all windings around the same core, not only in the primary.
The regulator IC decides when the output voltage in the capacitor has dropped enough for a new pulse. The output voltage really swings few millivolts up and down.