Thevenin's theorem in transistor circuit

Perhaps all you need is some visual aid. Here's your circuit and its equivalent version once I cut the top 22V rail in the middle. (You have to ask yourself why I can cut the circuit between those upper nodes when they both are at the same voltage).

And now I can redraw it so that I have a black box with one port. You can apply Thevenin to that one port and get your final circuit

It's as easy as that.

Thevenin's transformation is applied only to the part of the circuit consisting of the 22V source and the voltage divider (R1 and R2).

Anything else is left as it was; especially the fact that R3 is connected to a 22V source. Therefore a 22V source must still be present in the simplified circuit.