  Using the VINTERGATAN cosmological zoom simulation, we explore the
contributions of the in situ and accreted material, and the effect of galaxy
interactions and mergers in the assembly of a Milky Way-like galaxy. We find
that the initial growth phase of galaxy evolution, dominated by repeated major
mergers, provides the necessary physical conditions for the assembly of a
thick, kinematically hot disk populated by high-[$\alpha$/Fe] stars, formed
both in situ and in accreted satellite galaxies. We find that the diversity of
evolutionary tracks followed by the simulated galaxy and its progenitors leads
to very little overlap of the in situ and accreted populations for any given
chemical composition. At a given age, the spread in [$\alpha$/Fe] abundance
ratio results from the diversity of physical conditions in VINTERGATAN and its
satellites, with an enhancement in [$\alpha$/Fe] found in stars formed during
starburst episodes. Later, the cessation of the merger activity promotes the in
situ formation of stars in the low-[$\alpha$/Fe] regime, in a radially
extended, thin and overall kinematically colder disk, thus establishing
chemically bimodal thin and thick disks, in line with observations. We draw
links between notable features in the [Fe/H] - [$\alpha$/Fe] plane with their
physical causes, and propose a comprehensive formation scenario explaining
self-consistently, in the cosmological context, the main observed properties of
the Milky Way.
