There are a lot of different types of poly relationship structures and different names for them. The base unit of relationship is a standard couple where 2 people are together. Add another person in and they can either be in a relationship with only one of those people and form a "hinge" aka "V" or be in a relationship with both of those people and form a "triad" aka "throuple". As many people as those involved consent to can be added this way.

Most of the time it's one person who is in a relationship with multiple people who are each in relationships with multiple people. This forms a "polycule". Where you have the people you're in relationships with aka your "paramours" and they have the people they're in relationships with aka your "metamours". This group of relationships can take many forms and can be drawn out into a cool diagram like a molecule, hence the name polycule.

The people you're in a relationship with can break up with you like in any other relationship and vice versa. It's more complicated when you add in housing situations if you're all living together, multiple people are all dating each other, or if two people are married.

Using one of my breakups as an example:
I've been in a triad where one person broke up with the other. I was then put in the middle of their breakup drama. I set a boundary of not wanting to deal with their drama/shit talking of the other. One of them kept breaking that boundary, so I broke up with that person while still being in a relationship with the other. Luckily I was living with the person I stayed with or that would've been way more complicated.

gets voted out

slowly takes out an immunity totem i found in a charity shop.

Imagine getting broken up with by 2 people, both with non-binary haircuts. I'd probably jump into a river and become a trout

They mostly don't. Poly people think they do, but you see far, far more relationship volatility in polyerotic relationships than you do in monogamous.

Edit: I see that I'm getting downvoted by the people that are in non-monogamous relationships. Fact is that when you talk to sex-positive sex and relationship counselors, they will almost universally say that functional polyerotic relationships are the equivalent of post-doctoral work, while most people have relationship abilities equivalent to a barely-literate middle school level. It's not that multiamorous relationships are bad or wrong, or that the people that engage in them are wretched examples of humans (...although there are certainly more than a few of those) or anything like that, but to be functional that type of relationship requires a far greater level of self-awareness and honesty than most people are capable of. Hence the reason that they tend to be so volatile; more moving parts, more chances to fuck up.

In my personal experience I have found that most multiamorous relationships are more casual and less emotionally intimate (e.g., more shallow) than monogamous relationships. The people I have personally observed, including my own partners, have had less time to spend with any single person, and were more likely to jettison relationships rather than putting in the hard work to fix problems.

in case anybody who doesn't know, poly doesn't mean everyone is dating each other. Someone in a poly relationship can date someone who has no interest in dating their other partners. ofc a good rule of thumb is that everyone in this metaphorical web should be able to sit down and have dinner with each other without being mean or violent with each other.

It is pretty rare for my partners to date each other, so most breakups are usually “normal“. Even when they do, one breakup only concerns the two people involved, unless something really bad prompted it, which has never happened to me directly.

Is there a poly equivalent of something like the Magna Carta?

I wish Caprica wasn't cancelled. I liked their portrayal of a poly group.