We provide comprehensive experimental data and technology computer-aided design (TCAD) simulations to clarify total-ionizing-dose mechanisms in 16-nm Si FinFETs. In n-channel FinFETs irradiated to ultrahigh doses, the transconductance evolution rebounds (increase up to 3-10 Mrad followed by a decrease), while the drain-to-source leakage current steadily augments until reaching a plateau at very large doses. These effects result from positive charge trapping deep in the sidewalls of the shallow trench isolation (STI) and negative trapped charge accumulation localized in the upper STI corners. Larger sizes of inter-fin STI enhance the leakage current degradation of transistors with smaller numbers of fins. Hydrogen-induced border- and/or interface-trap generation at the Si/oxide interface at the STI corners leads to increased low-frequency noise (LFN) at doses $>{10}$ Mrad(SiO2). These results show that the quality of oxides and interfaces in the upper region of the STI adjacent to the device channel is crucial for the tolerance to ultrahigh radiation of modern FinFET technologies.