Need a little clarification about this BJT common emitter circuit
One important component in the circuit is the output capacitor between RC and RL.

At power up it will charge up to the DC voltage present at the collector of the transistor and from then on will act as if it was a battery and keep a constant DC voltage across it.

When the transistor takes more current as a result of the input signal it will cause the voltage at the collector to drop and this will be transferred through the capacitor to the load RL.

Since the assumption is that the capacitor voltage remains constant the voltage at RL will go negative. (below ground)

Similarly if the transistor current reduces the voltage at the collector will go positive and this will be communicated through the capacitor to RL which will then go positive relative to ground.

In order for the voltage across the capacitor to be unvarying the time constant of the capacitor and RL must be much greater than the period of the signal being amplified.

This type of "capacitor coupling" is common in amplifiers where the DC component of a signal is unimportant such as in audio or RF amplifiers.

The capacitors at the input (C1) and on the emitter will also have been selected to be large enough that the voltage across them remains constant for a complete cycle of the signal.