Recently, microresonator-based dissipative Kerr soliton frequency combs ("soliton microcomb") have emerged as miniaturized optical frequency combs. So far, soliton microcombs have been realized in many CMOS-compatible material platforms including silicon nitride (Si3N4) that is widely used in CMOS fabrication of microelectronic circuits. For Si3N4 photonic integrated circuits (PIC), the optical losses are critical for building soliton microcomb devices with high power efficiency, low power budgets and compact form factors. This PhD thesis addresses several key topics in the development of ultralow-loss waveguides and microresonators based on Si3N4 PIC, critical to integrated soliton microcomb technology. Through careful design and simulations of Si3N4 PIC, systematic development of Si3N4 wafer-scale fabrication process, and comprehensive characterization of the final Si3N4 devices, a novel Si3N4 fabrication technology featuring ultralow loss, engineered dispersion and wafer-level yield has been realized. Currently, integrated Si3N4 microresonators of Q factors exceeding 30 million (corresponding to a linear optical loss of 1 dB/m) have been routinely achieved. Meanwhile, thermal characterization using a microresonator modulation response measurement which is self-calibrated via the Kerr nonlinearity reveals that, the intrinsic absorption-limited Q factor exceeds 1 billion. Using these Si3N4 devices, integrated soliton microcombs with unprecedentedly low power budgets and repetition rates down to 10 GHz have been demonstrated, which allow highly compact soliton microcomb modules via hybrid integration with III-V lasers and new applications such as low-noise microwave synthesis. Furthermore, via monolithic integration with aluminium nitride, on-chip piezoelectric actuation of soliton microcombs with megahertz bandwidth has been realized, which can facilitate the realization of soliton-based massively parallel coherent LiDAR. Further beyond the domain of soliton microcombs, this thesis also contributes to other research topics such as the backward stimulated Brillouin scattering in Si3N4 and supercontinuum generation for mid-infrared dual-comb spectroscopy. Transferring this Si3N4 photonics technology developed during this PhD thesis to standard commercial foundries, and merging it with silicon photonics using heterogeneous integration technology, could significantly expand the scope of todayâs integrated photonics and seed new applications.