  We discuss the current status of lepton flavour universality observables in
semileptonic charged and neutral current $B$ meson decays. We recapitulate the
current experimental situation. Assuming the current tensions with the SM are
of beyond the Standard Model origin we present their implications for the
effective theory couplings. Then we discuss general leptoquark models before
finally focussing on the model with two scalar leptoquarks ($R_2$ and $S_3$)
which have couplings connected by the underlying Grand Unified Theory
framework. We present the viability of the model with regard to lepton flavour
universality and present opportunities to search for this model in future
high-intensity and high-energy experiments
