What is the biotechnology in Surrogate mothers ?

I'm just curious to know what's the biotechnology in the process of a surrogate mother. Do they just put the sperm inside the women or is it a fertilized egg put in tge 3rd party in which case it's the female who volunteer to be a vessel of that egg?What is the biotechnology in Surrogate mothers ?
The surrogate may or may not be the egg donor. fertilization takes place outside of the body in a petri dish (ie "in vitro") %26amp; then the viable embryos are implanted into the surrogate's uterus.

the surrogate is usually paid. a friend or family member can volunteer if they like but Surrogacy is a business and the women are paid for carrying the infants to term.