Probing the formation of ultrastable metallic glass from structural heterogeneity

•The structure underlying the unexpected stability of Zr-based TFMGs was tracked.•MGs with greater heterogeneity have a higher potential to form SMGs.•Slower deposition rates are required for ultrastability in more heterogeneous MGs.•The relaxation mechanism changes as the deposition rate decreases.•The geometry and distribution of loosely packed phases determines ultrastability.