Here, the buffer is NH₄Cl + NH₃
The strong base is added to the buffer solution, then the salt reacts with base to form weak base and water, which does not alter the pH
NH₄Cl + CsOH → NH₃ + CsCl + H₂O
NH₄⁺ + OH⁻ → NH₃ + H₂O
When we add a strong acid to the buffer solution, the base NH₃ reacts with acid and forms salt, which does not alter the pH
NH₃ + H⁺ → NH₄⁺
