I have heart for the NG only
such that the bleed valve had to be open to allow air to the start valve
Not sure what you mean,,,during start,air from whatever source you are using(APU,ground cart) is directed directly to the starter motor(some small turbine kind of motor) via the start valve.
The motor is then turning the n2 via the gear box,mechanically.
After cut out,the engine is self sufficient,lpt drives lpc etc....