Cooperative phenomena emerge in condensed matter when interactions between the constituent particles significantly impact, or even govern, the quantum dynamics of the system. The system then exhibits phases and properties that are undescribable by single-particle theories. For understanding the physics behind cooperative phenomena, it is important to disentangle the interplay between different degrees of freedom, including spin, charge, lattice, and orbit. In this dissertation work, we demonstrated that terahertz (THz) magnetospectrosocpy is a powerful tool for probing and elucidating cooperative phenomena in solids by studying four different types of quantum materials. First, we observed a narrow-band THz gain peak in a dense two-dimensional magnetoexciton gas in a photoexcited semiconductor quantum well. Second, by using THz polarimetry, we probed the collective magnetooptical response of surface carriers in a topological insulator in pulsed high magnetic fields. Third, we detected singular charge fluctuations in a quantum critical heavy-fermion metal in the form of ``ω/T-scaling" in THz conductivity. Finally, we discovered that the exchange coupling between two spin systems in a magnetic insulator follows the scaling behavior expected for Dicke cooperativity, a well-kown many-body process in quantum optics. The experiments and analyses in these studies can be extended to a variety of other quantum materials for advancing the understanding of many-body physics.