It is a long read yes but it is worth it now this isn't something that you should be taking a whole truth because it may not be but it is something to consider and would help increase your understanding of the astral plane and astral body.
from my understanding there are multiple planes of existence astral(synonymous with spirit plane?), spirit(astral plane too?), physical, psychic, and others(depending on personal beliefs) in these different planes there are different formations of energy (the creatures and unidentified "paranormal" animals) they can project into this world and stay here for a time but have to return, the astral plane is a plane of energy that the astral body(spirit?) travels to and is modeled after yours and others peoples knowledge of earth and what is happening on it at the moment basically a giant pool of knowledge given form, this could be the first "level" of the many "levels" there are here and the deeper you go the less it looks like earth and more like a foreign world, the problem with going too deep is that if you haven't gotten familiar with the area you are in before advancing you can get "lost" and in essence leave a husk of a physical body but most people are wary of such things and don't try to push things way past their limits or they get "stolen" by other beings demons or jealous spirits that crave life and then they become different people after that other has come back through the body, the astral body(spirit body too?) is one of the many bodies you have for the planes of existence, all work together and alone on the physical plane, but the astral body is another state of consciousness and body form that you enter and are able to experience that plain of existence for a short time because the physical body has many things to do and needs attention to survive, when you project into the astral plane you are moving your consciousness mainly out of the physical plain and into the astral plain of existence in other words you left the body and are visiting a new place and it is probable that you will encounter some beings there that are either good or bad
hope it helps