The circumgalactic medium (CGM) and intergalactic medium (IGM) play central roles in the baryon cycle by regulating gas accretion, feedback, recycling, and metal transport around galaxies, yet their diffuse nature makes their spatial distribution and kinematics difficult to observe directly. My dissertation uses giant optically emitting quasar nebulae, observed with MUSE, to study the morphology, ionization, and kinematics of this gas around quasars. I show that these nebulae arise through multiple pathways, including galaxy environments, interactions, and AGN feedback, making them powerful probes of the multiphase CGM and the coupled evolution of quasars and galaxies.