Recent integral-field spectroscopy observations have revealed that thick- and thin-disk star-formation histories are regulated by the interplay of internal and external processes. We analyze stellar-population properties of 24 spiral galaxies from the AURIGA zoom-in cosmological simulations, to offer a more in-depth interpretation of observable properties. We extracted edge-on maps of stellar age, metallicity and [Mg/Fe] abundance, and star-formation and chemical-evolution histories of thin and thick disks. These show signs of the interplay between internal chemical enrichment and gas and star accretion. Thick disks show particularly complex stellar populations, including an in-situ component, formed from both enriched and more pristine accreted gas, and a significant fraction of ex-situ stars.