String Theory in 4D is highly predictive due to the compactification of extra dimensions: specifying a set of abstract compactification parameters, the particle content, all symmetries and all interactions in the effective low-energy theory are uniquely fixed. This is in contrast to Quantum Field Theory: the effective Lagrangian from String Theory cannot be altered at will, i.e. no particles, symmetries or interactions can be added or removed by hand. Moreover, modular symmetries are a genuine stringy effect which can yield important consequences for the 4D effective theory. In this respect, we will discuss the unification of flavor symmetries, modular symmetries and CP from String Theory.