How do you get from the simplified transistor model to the one with two resistors and a constant current source? The AC signal generator seemingly just goes away.

For all models, there's always an AC generator at the input.

The slightly more complicated AC transistor model that we covered in Lecture 14 is shown in Figure 4.18 of Eggleston. In this model, the input sees an effective extra AC voltage $v_{ce}$ with proportionality constant $h_{re}$, the ``voltage feedback ratio'' or ``reverse voltage ratio'', in series with it. However $h_{re}$ tends to be a very small number ($\sim 10^{-4}$). So often we can just drop this term in our description of what the input sees and still get a reasonable answer. This is the simplified transistor model that Eggleston uses (and that we will also primarily use going forward). This model, with the $h_{re} v_{ce}$ component shorted out, is shown in Fig. 4.17.