Theoretical investigations for observables are crucial to answer fundamental questions at the future electron-Ion Collider. In this talk, I will discuss our recent theoretical works in calculating cross-sections and substructure for jets and open heavy flavor in electron-nucleus collisions. For jet production, we demonstrate theoretically how to disentangle the effects from nuclear parton distribution functions and the ones that arise from strong final-state interactions between the jet and the nuclear medium. For open-heavy flavor hadron production, we show how to identify the optimal observables, center-of-mass energies, and kinematic regions most sensitive to the physics of energy loss and hadronization at the EIC.