The Cosmic Microwave Background, the oldest light in the universe, provides a powerful tool to test the theory of inflation and constrain the standard ΛCDM cosmological model. The theory of inflation describes a period of rapid, exponential growth in the early Universe. A measurement of r, the tensor-to-scalar ratio of CMB polarization, would provide evidence for inflation and measure its energy scale. However, this is an extremely difficult measurement to make due to the faintness of the signal. Historically, Small Aperture Telescopes (SATs) have been used to constraint inflation. However, there is immense power that can be gained by combining the low-ℓ (large angular scale) measurements from Large Aperture Telescopes (LATs), such as the South Pole Telescope (SPT), with SATs.

The first part of this dissertation describes the design, characterization, and deployment of the SPT-3G receiver on the SPT. There is special emphasis placed on the development and characterization of the frequency-multiplexed readout, and characterization of system crosstalk. The second part of this dissertation uses data from the 2019 and 2020 austral observing season over a 1500 deg² patch of sky to make low-noise maps of the B-mode polarization of the CMB. This delves into techniques used for mitigating low-ℓ noise sources, including polarized atmosphere. These include weighting schemes, and an unbiased method to remove this noise source from data utilizing co-pointing detectors of different frequencies and the fixed spectral scaling of polarized atmosphere in accordance with Rayleigh scattering. This work serves both as demonstration of a LAT achieving high sensitivity at low-ℓ and as a study of atmospheric noise that will inform the design and operation of future LAT and SAT telescopes. This shows a path for producing competitive r constraints from a LAT optimized for high-ℓ science, and is a proof-of-concept for using LAT data in the search for primordial gravitational waves with next-generation CMB experiments.