Classification of solids and their heat transport mechanisms. The left panel categorizes solids based on atomic disorder, ranging from simple and complex crystals through partially disordered solids to glasses. The corresponding vibrational power spectra illustrate the transition from well-separated phonon modes in simple crystals to overlapping modes in complex crystals due to phonon broadening. The right panel depicts the typical temperature ($T$) dependence of lattice thermal conductivity (${\kappa }_{\text{L}}$) across different solid categories, highlighting the transition from ${\kappa }_{\text{P}}$-dominated transport in simple crystals to ${\kappa }_{\text{C}}$-dominated transport in disordered glasses.