Defect center-based spin qubits in solid-state materials show great promise as quantum sensors and nodes in quantum networks. The success of these applications relies on precise control and understanding of the qubit host material and noise environment, which ultimately dictate qubit coherence. The nitrogen vacancy (NV) center in diamond is a particular defect qubit with a robust spin-photon interface and long spin relaxation times, enabling a range of advances in quantum information science. However, open questions remain regarding the surrounding host material. In particular, while it is possible to routinely grow single crystal diamond with nitrogen doping, to synthesize NV centers, a reliable method for characterizing this doping under growth parameters relevant for many NV applications is lacking. Furthermore, unconverted nitrogen spins (P1 centers) constitute a major source of decoherence in the diamond lattice. While P1 center spin properties have been studied in bulk, there are few experiments that probe single P1 centers. Characterizing single P1 spins will advance understanding of P1-induced decoherence as well as aid in the implementation of P1 centers as auxiliary qubits in applications of the NV center. In this thesis, I will describe our recent studies of interactions between the NV center and nearby P1 electron spins. After an introduction into quantum science and engineering in Ch. 1, I describe the two defect centers in Ch. 2. I then lay the groundwork for the studies in this thesis from two very different perspectives: In Ch. 3 I review a theoretical treatment of noise and spin baths; in Ch. 4 I discuss the details of plasma-enhanced chemical vapor deposition (PE-CVD) diamond growth and NV center synthesis that we implement in our studies. The results in Ch. 5 and Ch. 6 require an appreciation of these two disparate approaches to the systems we study. In Ch. 5 I present quantitative computational studies of NV decoherence at the length and density scales relevant for synthesizing single NV centers, providing a reference for future NV synthesis, as well as revealing coherence behavior dependent on the spin bath dimensionality. These data are then applied to characterize nitrogen density in-situ via a statistical model, bypassing the need for unreliable bulk characterization techniques. In Ch. 6 I present measurements of spin bath dynamics at the single-spin level as a means to understand microscopic processes underlying central spin decoherence. I describe a polarization- and time-resolved measurement technique of a strongly coupled NV-P1 system that enables a probe of P1 polarization decay under arbitrary microwave and optical drives. These measurements reveal decay mechanisms on the single-spin level, allowing us to address open questions about the behavior of P1 spin baths in diamond.