Tony,
I got a chuckle out of your questions, not because they were bad, but because there are not simple easy answers and the topic is not part of our thread subject. Do some searches and you’ll find your answer. The fact that you are on an Ardupilot forum may be a good start. Here are some threads from a few years ago.
PX4 vs ArduPilot - when to choose what
Should I use PX4 flight control stack or the APM 3.3 for Pixhawk?
Rollys is our resident PX4 expert and often shares his experiences. Without him, I would know much less about the PX4 stack and implementation. Perhaps he has some links to share.
Cheers!