The high degree of control on ultracold gases allows us to precisely manipulate their internal state. When the gas is made of atoms in two different internal states, it can be considered as a two-component spin mixture. Below a critical temperature, the gas becomes a superfluid mixture, never realized before with any other platform, and therefore interesting to study per se, but it also constitutes a promising and versatile platform for applications in spintronic devices or to study phenomena belonging to very different fields, such as magnetism, high-energy physics or gravitation. Here, I will revisit ground-state properties and excitations of a binary bosonic superfluid, and then introduce a coherent coupling between the states and treat the global state of the atoms as a spin in the presence of a variable external field.