The availability of thin-film lithium niobate on insulator (LNOI) and advances in processing have led to the emergence of fully integrated LiNbO3 electro-optic devices. Yet to date, LiNbO3 photonic integrated circuits have mostly been fabricated using non-standard etching techniques and partially etched waveguides, that lack the reproducibility achieved in silicon photonics. Widespread application of thin-film LiNbO3 requires a reliable solution with precise lithographic control. Here we demonstrate a heterogeneously integrated LiNbO3 photonic platform employing wafer-scale bonding of thin-film LiNbO3 to silicon nitride (Si3N4) photonic integrated circuits. The platform maintains the low propagation loss (<0.1 dB/cm) and efficient fiber-to-chip coupling (<2.5 dB per facet) of the Si3N4 waveguides and provides a link between passive Si3N4 circuits and electro-optic components with adiabatic mode converters experiencing insertion losses below 0.1 dB. Using this approach we demonstrate several key applications, thus providing a scalable, foundry-ready solution to complex LiNbO3 integrated photonic circuits.