In this thesis, various theoretical results surrounding the quark--gluon plasma which is created in the ultrarelativistic collisions of ions are presented. The many-body, strong-coupling, and real-time characteristics of those collisions partially exclude first-principle quantum chromodynamics calculations, whereas the effective theory of hydrodynamics allows for a rather successful description of the quark--gluon plasma. A criterion of hydrodynamic stability is used to restrict the transport coefficient bulk viscosity. In addition, various collisional systems over a wide range of energy are successfully modeled for particle spectra, yields, flow coefficients, and HBT radii. Despite questions of hydrodynamic applicability to few-particle systems, there is reasonable agreement with experimental data for p+p collisions in this framework. Notably, the strong dependence on the bulk viscosity of simulation results for p+p collisions could help constrain this transport coefficient. All of those aspects solidify our quantitative understanding of the quark--gluon plasma produced in ion collisions
