  The origin of thick disks and their evolutionary connection with thin disks
are still a matter of debate. We provide new insights into this topic by
connecting the stellar populations of thick disks at redshift $z=0$ with their
past formation and growth, in 24 Milky Way-mass galaxies from the AURIGA
zoom-in cosmological simulations. We projected each galaxy edge on, and
decomposed it morphologically into two disk components, in order to define
geometrically the thin and the thick disks as usually done in observations. We
produced age, metallicity and [Mg/Fe] edge-on maps. We quantified the impact of
satellite mergers by mapping the distribution of ex-situ stars. Thick disks are
on average $\sim 3$~Gyr older, $\sim 0.25$~dex more metal poor and $\sim
0.06$~dex more [Mg/Fe]-enhanced than thin disks. Their average ages range from
$\sim 6$ to $\sim 9$~Gyr, metallicities from $\sim -0.15$ to $\sim 0.1$~dex,
and [Mg/Fe] from $\sim 0.12$ to $\sim 0.16$~dex. These properties are the
result of an early initial in-situ formation, followed by a later growth driven
by the combination of direct accretion of stars, some in-situ star formation
fueled by mergers, and dynamical heating of stars. The balance between these
processes varies from galaxy to galaxy. Mergers play a key role in the mass
assembly of thick disks, contributing an average accreted mass fraction of
$\sim 22$\% in the analyzed thick-disk dominated regions. In two galaxies,
about half of the geometric thick-disk mass was directly accreted. While
primordial thick disks form at high redshift in all galaxies, young metal-rich
thin disks, with much lower [Mg/Fe] abundances, start to form later but at
different times (higher or lower redshift) depending on the galaxy. We conclude
that thick disks result from the interplay of external processes with the
internal evolution of the galaxy.
