  The Simons Observatory (SO) will observe the cosmic microwave background
(CMB) from Cerro Toco in the Atacama Desert of Chile. The observatory consists
of three 0.5 m Small Aperture Telescopes (SATs) and one 6 m Large Aperture
Telescope (LAT), covering six frequency bands centering around 30, 40, 90, 150,
230, and 280 GHz. The SO observations will transform the understanding of our
universe by characterizing the properties of the early universe, measuring the
number of relativistic species and the mass of neutrinos, improving our
understanding of galaxy evolution, and constraining the properties of cosmic
reionization. As a critical instrument, the Large Aperture Telescope Receiver
(LATR) is designed to cool $\sim$ 60,000 transition-edge sensors (TES) to $<$
100 mK on a 1.7 m diameter focal plane. The unprecedented scale of the LATR
drives a complex design. In this paper, we will first provide an overview of
the LATR design. Integration and validation of the LATR design are discussed in
detail, including mechanical strength, optical alignment, and cryogenic
performance of the five cryogenic stages (80 K, 40 K, 4 K, 1 K, and 100 mK). We
will also discuss the microwave-multiplexing ($\mu$Mux) readout system
implemented in the LATR and demonstrate the operation of dark prototype TES
bolometers. The $\mu$Mux readout technology enables one coaxial loop to read
out $\mathcal{O}(10^3)$ TES detectors. Its implementation within the LATR
serves as a critical validation for the complex RF chain design. The successful
validation of the LATR performance is not only a critical milestone within the
Simons Observatory, it also provides a valuable reference for other
experiments, e.g. CCAT-prime and CMB-S4.
