Stellar populations of galactic disks are an important tracer of the evolution history of disk galaxies, from the early stages when older thick disks formed, to recent times when thin disks formed most of their stars. Thus, different evolution stages of a disk galaxy are encoded in their vertical structure, making observations of edge-on galaxies ideal to unveil them. However, early studies mostly based on long-slit spectroscopy did not agree on how thick disks formed and on their evolutionary connection with thin disks. Only the advent of MUSE high-spatial-resolution integral-field spectroscopy revolutionized this field, thanks to spatially resolved stellar ages, metallicities and alpha enhancements. I will summarize my work on this topic, based on several high-quality MUSE datasets, including data from the Fornax 3D and the GECKOS surveys. These results revealed that nearby thick disks are made up of different stellar populations in different types of galaxies, suggesting that they result from different evolution histories. The sharp transition between thick and thin disks in earlier-type disk galaxies suggests that they formed in two distinct evolution phases. Thick disks formed at very early times and in a fast timescale. Very late-type galaxies, with high star-formation rates, show little differences between relatively young thick and thin disks, suggesting a more continuous and later upside-down formation, and a slow extended mass assembly for thick disks.