Collective flow, its anisotropies, and event-to-event fluctuations in relativistic heavy-ion collisions are reviewed, focusing on extracting the specific shear viscosity of the quark-gluon plasma (QGP) from experimental data. The similarities between the Big Bang and heavy-ion collisions are emphasized. The review discusses relativistic fluid dynamics, equation of state, transport coefficients, and the role of initial-state fluctuations in shaping collective flow. It highlights the importance of dissipative effects and the transition from ideal fluid dynamics to more realistic models incorporating viscosity and microscopic interactions. The paper explores the relationship between initial density fluctuations and anisotropic flow, the role of shear and bulk viscosity in determining flow patterns, and the extraction of QGP viscosity from experimental data. It also discusses the evolution of radial and anisotropic flow, the impact of initial-state fluctuations on final observables, and the use of hybrid models combining fluid dynamics with microscopic hadron cascade simulations. The review concludes with the significance of event-by-event fluctuations in determining the QGP transport coefficients and the importance of high-precision measurements in understanding the properties of the QGP.Collective flow, its anisotropies, and event-to-event fluctuations in relativistic heavy-ion collisions are reviewed, focusing on extracting the specific shear viscosity of the quark-gluon plasma (QGP) from experimental data. The similarities between the Big Bang and heavy-ion collisions are emphasized. The review discusses relativistic fluid dynamics, equation of state, transport coefficients, and the role of initial-state fluctuations in shaping collective flow. It highlights the importance of dissipative effects and the transition from ideal fluid dynamics to more realistic models incorporating viscosity and microscopic interactions. The paper explores the relationship between initial density fluctuations and anisotropic flow, the role of shear and bulk viscosity in determining flow patterns, and the extraction of QGP viscosity from experimental data. It also discusses the evolution of radial and anisotropic flow, the impact of initial-state fluctuations on final observables, and the use of hybrid models combining fluid dynamics with microscopic hadron cascade simulations. The review concludes with the significance of event-by-event fluctuations in determining the QGP transport coefficients and the importance of high-precision measurements in understanding the properties of the QGP.