Primary partners, are they always a thing?

Not sure if this goes in curious/learning or musings so let me know if I need to change the flair here.

I'm also newer to terms and dynamics here, But in poly, there is generally an established partner and newer relationships from what I've seen. I also hear a lot of books and resources talk about a primary relationship, established partner or a long standing bf/gf/spouse or nesting partner etc that is the first person to be considered because the open relationship started with them and goes out from there. Ive also seen that there are also hierarchical systems out there and I've seen some people try to actively minimize the hierarchy systems in poly by making relationships more equal to one another.. but I also see here that a lot of people that have a husband/wife and a boyfriend / girlfriend dynamic. While those labels don't make the relationships anything lesser, I feel like there is a primary relationship that is typically considered first or is consulted before others? Can I get your guys's opinions on this? Your feelings, experiences? What you've seen?

Thanks

TLDR: is there always a primary partner in poly and is it always the first person you are with?