Lithium niobate (LiNbO3) is a key material in photonics due to its exceptional properties. The linear electro-optic effect, also known as the Pockels effect, in lithium niobate induces a linear change in the refractive index when an electric field is applied, enabling efficient light modulation. The wide transparency window makes the material suitable for photonic applications from the visible to mid-infrared spectra. These properties, along with commercial availability, make lithium niobate a crucial material for microwave photonic applications such as high-speed optical communications, radio-frequency filtering, distance measurements, etc. The integration of lithium niobate into photonic circuits has enabled the development of low-loss photonic circuits, high-quality entangled photon pair generation, and wavelength-division multiplexing, highlighting its versatility and efficiency in various photonic applications. Over the past years, integrated lithium niobate photonics have heavily relied on high-quality lithium niobate thin films on insulator (LNOI) technology, facilitating the development of nanophotonic waveguides and microphotonic modulators. However, lithium niobate photonic integration is still a developing field of research with multiple problems to be solved. This thesis reports on the development of a heterogeneously integrated lithium niobate photonic platform based on low-loss silicon nitride optical waveguides for microwave photonics applications. The work in this thesis covers the platform development chain from multiple aspects of photonic design and structure optimization to experiments in nonlinear optics and characterization of basic components such as electro-optic modulators.