Processes of formation of islands of individual components on terraces during step-flow growth of a multicomponent crystal are considered. This phenomenon is observed repeatedly in experiments and is possible when an excess of one of the components accumulates on the surface. In this case, multicomponent steps can be used as an additional tool to control the ensemble of islands on the crystal surface because, unlike single-component systems, it is possible to independently control the motion of the steps and the kinetics of the growth of one-component islands. Analytical criteria are obtained for identifying the conditions under which the nucleation of islands of individual components is possible. The interaction of an individual island and a step on the surface of a multicomponent crystal is investigated. A mechanism of dynamically stable forming quasiperiodic layers of islands is discussed. Under certain conditions, the observed phenomena can be used for the bottom-up formation of various quasi-ordered nanostructures both on the surface and in the bulk of the growing crystal.