Every poly configuration is unique and the people involved get to determine how that looks and what the parameters are. There are people who are in a "closed relationship" who are involved with more than one person long term and don't see anyone else. Most common is what's called Parallel Poly, where a couple who are dating, live together, or are maybe even married, each see other people separately. There are other types as well. I'm linking you a story that goes into that in more detail, but I'm always open to questions, so feel free.