What is RB in this CE circuit?
RB in the model is the effective parallel resistance of components external to the transistor.
In this case it will be the the parallel resistance of R1 and R2.
From the small signal point of view VCC and ground are the same so R1 and R2 are in parallel. It is assumed that the power supply has a low impedance for AC.
Note that RC, at the output, is also shown connected to ground where in the upper schematic it connects to VCC.