To build a push-pull amp you need to split the signal into two opposite phases and feed each phase to drive one half of the push pull output stage.
There are many forms of phase splitter to do this, the simplest is to note the collector and emitter or source and drain outputs from a transistor appear as two opposite phases.
The two collector or drain outputs from a long tail pair also form a push-pull system.
I was in Edinburgh over the weekend.
Cold, wet and blustery, but the St Andrew's celebrations were good.