Prepeak in the structure factor of alcohols is known for a half century and was attributed to one of two mechanisms (i) self-assembly in aggregates and (ii) existence of spatial heterogeneity. Although both expinations are often argued the molecular origin is yet unclear. In this work, molecular dynamics simulation of neat alcohols and their mixtures in the presence of an apolar liquid in bulk and in confined phases is performed to unveil and to clarify the origin of the prepeak at the molecular scale. Unambiguously, we show that the existence of the prepeak is the result of the self-assembly in clusters leading to long-range correlations rather than the spatial heterogeneity. We also establish that the confinement of neat liquids at the nanoscale does not erase the clustering and the prepeak but strongly reduce the spatial heterogeneity. Regarding the binary alcohol/toluene mixtures, we highlight the possibility to erase the clustering and the spatial heterogeneity from nanoconfinement inducing the formation of a core-shell structure. By tuning the interfacial chemistry and pore size, we shed light on the possibility to control the spatial heterogeneity, the self-assembly, and the microphase separation.