Mixing fraction (f_GC) probability distribution function. A mixing fraction f_GC = 0 (f_GC = 1) means that all BBHs are born from isolated binary systems (GCs). In the upper and lower panels, we and do not marginalize over the spin parameters χ_eff and χ_p in our analysis, respectively. All the models presented in this paper are shown with different colors, as described in the legend.