As a byproduct, digestate liquid derived from food waste anaerobic digestion (DFW) offers a promising hydrothermal medium due to its inherent alkalinity, high nitrogen content, and abundance of reactive organic species. Although DFW has proven effective in improving hydrochar characteristics, its mechanistic impact, especially on the often-overlooked non-carbonized fraction, remains insufficiently understood. This study systematically investigates the feedstock- and solvent-dependent mechanisms of hydrothermal carbonization (HTC) using structurally representative biomass precursors in both deionized water and digestate liquid. Combustion analysis revealed that interactions between digestate-derived proteins and carbohydrates enhanced the reactivity of hydrochars, reducing ignition temperatures by 10 % for lignin and 13 % for cellulose. A multiscale approach combining FTIR-XPS analysis showed that digestate promoted depolymerization and decarboxylation, resulting in higher carbon content (∼69 %) and lower O/C ratios (<0.2). Simultaneously, digestate facilitates the formation of oxygen-rich surfaces and promotes nitrogen doping (>3.0 %) while enhancing the structural integrity of the carbon core. Mechanistic differentiation revealed solvent-specific pathways: DFW promoted keto-enol tautomerism, whereas sugars in deionized water primarily followed furfural-type degradation. Both pathways contributed to carbon microsphere core formation. Notably, digestate reduces feedstock-dependent variability in hydrochar composition and structure. By combining real-waste media with model precursors, this study reveals new insights into hydrochar structural evolution and offers a scalable strategy for producing tailored carbon materials from heterogeneous biomass.