Balan says, "The measure of all right angles cannot be equal as their arms can be of different lengths." Why is Balan's statement not true?

The $9$ steps in paper plane making are given in a catalogue.

Lets expand the figure for Step $\mathrm{III}$, as shown below:

If in the given figure, $\angle 1=\angle 2$ and $\angle 3=\angle 4$, and you have to prove that $\angle \mathrm{ABF}=\angle \mathrm{CBF}$, then state the Euclid's axiom which is to be used here.