The Cosmic Microwave Background (CMB) carries the imprints of the conditions in the early universe and the large-scale structure it has traversed as it travels to our telescopes. Thus, the CMB serves as a window into the physics of our universe allowing measurements of primordial gravitational waves, large scale structure formation, the sum of neutrino masses, dark energy, dark matter, and fundamental physics. Achieving these ambitious science goals requires equally ambitious instrumentation. The Simons Observatory, a ground-based CMB experiment, is designed to observe the CMB temperature and polarization signals to an unprecedented sensitivity. A combination of one Large Aperture Telescope (LAT) and three Small Aperture Telescopes (SATs) will measure the temperature and polarization anisotropy of the CMB with ~60,000 background noise-limited detectors operating at ~100mK, covering frequencies from ~20 to ~300GHz. To measure the tiniest signals of the CMB, we must control optical systematics and advance detector sensitivity to unprecedented levels. I present techniques to control optical systematics, design highly sensitive detectors and feedhorn arrays, and characterize their performance. I present the design and performance of the optical properties of novel microwave absorbers, and beam map a complete cryogenic optical system. I present software to model these systems, including hardware imperfections, demonstrating how systematics propagate into measurements; the software uses machine learning to correct such systematic errors by correcting alignment of mirrors. Lastly, I present a measurement of the Atacama Cosmology Telescope (ACT) beam that is central to the ACT Data Release 6 (DR6). This beam measurement is made possible with a novel approach for characterizing the on-sky beam by stacking point sources. Understanding the beam of the instrument is critical for characterizing the angular response of the instrument, and therefore critical for achieving the ambitious measurement goals for inflationary signals and light relativistic species in ACT and other future experiments.