You can only get HIV from someone or something already infected. This includes humans, needles, breast milk, etc. HIV virus must come in contact with a damaged or open tissue.
You can get or transmit HIV only through specific activities. Most
commonly, people get or transmit HIV through sexual behaviors and
needle or syringe use.
You must have the infection to transmit it. Multiple partners increase risk, but only transmit if one partner already has infection.
How does a person get HIV? From someone or something already infected. If we go back to the first person infected then:
They believe that the chimpanzee version of the immunodeficiency virus
(called simian immunodeficiency virus or SIV) most likely was
transmitted to humans and mutated into HIV when humans hunted these
chimpanzees for meat and came into contact with their infected blood.