Cosmological zoom-in simulation of a MW-like galaxy. Only stars that formed within the main halo are shown, excluding those accreted from dwarf galaxies. The left column displays the spatial distributions in an edge-on X-Z projection at z = 0, divided into four age bins. The right column shows the corresponding phase-space distributions of star particles in the [Fe/H]–L_Z space, selected from a region near the solar circle (6 < R < 20 kpc and 2 < |Z|< 10 kpc). Star particles with ages between 7.9 and 9.9 billion years (middle panels) originated during a burst of star formation following the major merger at z ∼ 2. Stars formed before this period (top panels) exhibit characteristics similar to the heated-disc population in the MW, whereas younger stars (bottom panels) contributed to the disc’s inside-out growth.