The 6V does not need to be a sqaure wave. You can see in the waveform I posted that it is just a rising edge at 100ms (blue trace) and that it remains a constant 6V after that. All oscillators need something to get them started. In real circuits, that can be a switch on transient, or just random noise. Usually, something is there for starting. I've seen other oscillators that include a starting circuit, but these are rare. If you build this circuit, it will probably start correctly. But in simulations, you might need some kind of transient to make it start. Until now, I haven't simulated an oscillator, so I had to learn that too.