The output resistance is a parameter used to reflect the characteristics of the amplifier circuit. The output resistance is the output resistance and has nothing to do with the load resistance. When the amplifier circuit is regarded as a dual-port network, ro is not related to external conditions, and there is actually no such a resistance (rce is generally a resistance of tens of kΩ or more, in the three-pole management theory circuit model, it is They are connected in parallel at the output, so they are ignored. For the specific derivation, a full differential is enough). In order to reflect the dissipation of the amplifier's internal output circuit due to non-ideal conditions in the theoretical calculation, such a concept is introduced based on the output I-V characteristics. In layman's terms, it is "carrying capacity".
In the circuit topology, for the multi-stage amplifier circuit, the input resistance of each stage is the additional load resistance of the previous stage, and the load resistance and output resistance are not related.

The load resistance is related to the output resistance. For analysis and modeling calculations, the load resistance RL' of the amplifying circuit = the parallel connection of the external load resistance RL and the output resistance ro. Because dissipation is related to electric current, it is so stipulated. Just like a truck, when pulling goods, it must pull the car body to go together.