Abstract: The chemistry of interstellar clouds sets the critical initial conditions of the chemical processes in the late stages of star formation and is essential to completing the picture of the origins and evolution of biotic chemical complexity. This talk will introduce cutting-edge molecular identification techniques, processing dense and sparse spectra, to uncover very weak molecular signals that are often buried under noise. Leveraging high-sensitivity single-dish and interferometric radio observations performed with the Green Bank Telescope, Atacama Large Millimeter/Submillimeter Array, and Very Large Array, these techniques open up the possibility of discovering and characterizing complex molecules in exotic environments. In this talk, I will present the discovery of large isocyanide species in space as well as the new mechanistic insights into CN-vs-NC chemistry under prestellar dark-cloud conditions. In addition, with a focus on massive protostellar molecular clouds, I will discuss how distinct molecular spatial distributions help to constrain the formation mechanism and chemical relationship of the interstellar molecules, bring to light the potentially unresolved substructures of the source, and reveal the origins of the maser emission of complex molecules.