What is the reason behind choosing adiabatic and isothermal process for carnot engine?

My book states the following reason. I am not able to understand it. It would be great if you could put it in simpler words.

• A process is not quasi-static if it involves finite temperature difference between the system and reservoir. Hence, in a reversible heat engine operating between two temperatures,mheat should be absorbed and released isothermally.

• Employing any other processes other than adiabatic, say isochoric, to take the system from one temperature to another, we shall need series of reservoirs in the temperature range $T_2$ and $T_1$ to ensure that the process is quasi-static.

What is the reason behind choosing adiabatic and isothermal process for carnot engine?

My book states the following reason. I am not able to understand it. It would be great if you could put it in simpler words.

• A process is not quasi-static if it involves finite temperature difference between the system and reservoir. Hence, in a reversible heat engine operating between two temperatures, heat should be absorbed and released isothermally.

• Employing any other processes other than adiabatic, say isochoric, to take the system from one temperature to another, we shall need series of reservoirs in the temperature range $T_2$ and $T_1$ to ensure that the process is quasi-static.