Build PDA for a language with unknown input alphabet


$ L_1 ,L_2$ are regular language. We form a new language $ L_{12}$ as follows: $ L_{12}=\left \{ w_1\cdot w_2|w_1\in L_1\wedge w_2\in L_2\wedge|w_1|=|w_2| \right \}$

In this exersice I am not given any alphabet and I’m required to build PDA for $ L_{12}$ , but by definition $ M=\left \{Q,\sum,\Gamma ,\delta ,q_0,-|,F\right\}$ and I don’t have any alphabet to work with.By intuition if the alphabet is similiar can effect the solution than if it wasn’t similiar.