Massive galaxies (log M*>11.5 Msun) along with their dark matter halos are uniquely crucial probes to the galaxy formation process and the nature of dark energy. Predicted by the promising hierarchical formation scenario, the stellar halos of massive galaxies keep vital fossil records of their assembly history and galaxy-halo connections. However, due to its low surface brightness, much of this information is yet to be uncovered. In the last few years, thanks to deep imaging surveys such as the

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... one by Subaru Hyper Suprime-Cam (HSC), we can map the stellar mass distributions of low- to intermediate-redshift massive galaxies to their extreme outskirt at R>100 kpc, and reveal the subtle but intriguing structural diversities in their stellar halos. Taking advantage of the unprecedented weak lensing capability of the HSC survey, we further established an improved connection between the structure of the stellar envelope of a massive galaxy to the assembly of its dark matter halo. In light of these new results, we are about to gain significant and new insights about the formation of massive galaxies and dark matter halos when combining the much-improved imaging and lensing capability from the Nancy Grace Roman space telescope. I will present some of the scientific and technical lessons we learned from the HSC survey and discuss their implications in the era of Nancy Grace Roman.