Metal nanoparticles are omnipresent in today's applied and fundamental research. Both wet-chemical as well as physical procedures for their fabrication are well-established, where the latter is of particular interest as they supply surfactant-free particles. The particle growth has been investigated for several decades, but due to its complexity, involving kinetic and dynamic processes on various length and time scales, often only phenomenological rules of thumb are available. In this study, we report on bimetallic AgAu nanoparticles and demonstrate how the additional degree of freedom of the chemical composition can be used to derive information about how and where the particles grow, depending on two different cluster source types (hollow magnetron vs. laser vaporization). The chemical composition is quantified on the single-particle level using electron-induced x-ray spectroscopy (STEM-EDS) and shows significant differences for the two fabrication routes. Based on Molecular Dynamics and Monte-Carlo simulations, we derive that for hollow cylindrical sources both the mean particle size and the chemical composition are determined within the plasma region, where particles not only grow but also evaporate low-boiling silver. The comparably large plasma plays a decisive role here, as opposed to planar magnetron or laser vaporization sources, where no such evaporation is observed. These results shed light into the complex cluster growth and help understand and optimizing nanoscale fabrication processes.