Last question: So BLEED AIR is taken from high pressure stage or low pressure stage, depending on what we need...
But has the APU BLEED already got that air
Both the engine (the big one) you want to start and the APU (small guy) have compressors and bleed air. Since the APU is running it has air to spare. As said before it uses this air to spin a much smaller device (the starter) which through gears turns the big engine.
The big engine has no bleed air available during start and no direct connection exist in the air between either the APU (small), plus the starter (tiny) and the big engine.
A neat little schematic might focus your mind on this rather than more words