The AR-15 is a standardized Military Specification based platform. There are variations and customization from some vendors, but most receivers are standard.
There is a mounting bolt in the base of the hand grip that must generally be removed to disassemble the trigger assembly. You should be able to find any number of assembly videos on YouTube, or get a shop manual or government issued armorer's manual like:
IIRC the grip bolt holds captured a spring and indent pin involved with the safety / fire selector.