There is a "thread" connecting your astral body to your physical body and when the thread is severed , you simply disconnect from your physical body and you die..however , do not worry. The thread cannot be severed easily .
As for your second question , you do not breathe in the astral..The astral body is one of your non physical bodies and does not require air. You can also perform regular body movements. However , certain limitations you have are gone , such as in the astral plane , you can spontaneously manifest to a area , transform and fly.
The astral plane is extremely close to the physical world and even overlaps the phsycial. Hence , some thoughts and constructs you create such as servitators first resides there. Hence , if you imagined a pie or of some sort , it will appear yes , however , it will fade eventually..and so , it is not really "real"
You cannot alter the world in a sense where you can effortlessly turn on the television or touch objects or perhaps even wreck somebodies house (in case that was what you were thinking of XD), however , you can lower your vibrational state so that you are more "dense" and can be more easily sensed by physical beings. If low enough , you can have the ability to touch objects and to a extremely limited degree , move them.
Take note however , that if your vibrational state gets too low , you will automatically snap back into your physcial body. It is something tricky to perform , as it is hard to control your state to such a degree which is low enough to make you more "sensable" but not snapping back into your physcial body at the same time
As to your next question , yes , you can see spirits and other non physical entities when you astral project out of your physical body
You can talk to other non physical entities i suppose , but not physical people unless they are more "spiritually aware".
Also , you tend to encounter entities with a similar vibrational level such as yours , so if your vibrational levels are higher , you will be less likely to run to lower vibrational beings , though i suppose you can intentionally seek them out