Despite its successes, the standard model of cosmology is undeniably incomplete. Fundamental questions about the physics of the primordial universe, the nature of dark matter, and dark energy remain large gaps in our understanding of the universe. Meanwhile, recent discrepancies surrounding the growth of large-scale structure and the behavior of cosmic expansion hint at possible limitations with our current model. Observations of the sky at millimeter wavelengths provide us with access to distinct cosmological signals which uniquely probe different epochs of our cosmic history. The recent establishment of ground-based millimeter-wave observatories has enabled a new era of precision cosmology through measurements of the cosmic microwave background primary and secondary anisotropies across a broad range of angular scales. The deep maps produced by these millimeter-wave surveys encode a wealth of cosmology and fundamental physics, and could unveil new insights into the most profound questions about our universe. This thesis describes research efforts split across three spectral octaves and between two millimeter-wave observatories located in the Atacama Desert in Chile. First, I describe a power spectrum analysis of 27/39~GHz maps produced by the final-generation camera on the Atacama Cosmology Telescope, presenting the highest resolution and signal-to-noise measurements of the sky at these low frequencies. Next, I give an overview of the next-generation Simons Observatory large and small aperture telescopes, and detail my work in developing these instruments. These efforts include instrument optimizations, the design of high-frequency 220/280~GHz detector pixels, and in-lab performance validation of a fully-integrated mid-frequency 90/150~GHz camera module for the large aperture telescope. Finally, I will give a brief update on the deployment status of the Simons Obseratory and a look towards the near-future of millimeter-wave surveys.