SO and I have lived together for more than 30 years.
I have always owned the house. At first he paid rent - he worked out what he thought was fair, and I said it was too much, so we reached a compromise. At some stage, things changed, and he hasn’t paid rent since, but pays all utilities, and I pay rates (local government charges for the property - whatever you call them).
Where I live, everything is split down the middle if you break up after more than a year.
There are also tax implications - if you rent part of your PPOR you can claim rates and utilities off your income tax but add the rent as income. Your house is tax exempt unless you have rented it out for more than 6 years. You can see that the way we agreed to split actually had positive tax implications for us. So I suspect that different arrangements make more sense in different countries or states.
We are both FIRE. Neither of us have children. Both our jobs gave us a small pension, with half reversionary - but only to a spouse or someone who can prove de facto/co-dependance, so no one else is eligible. But if one of us died, and that family disputed our status, they could possibly get that person’s portion of our money and the reversionary part of the pension would be lost. As a result, we now have joint accounts where, until recently, we had completely separate finances. Even though some accounts are joint, we still think of the money as being separate, and use it independently.
We both intend the other to inherit everything, but to split it between both families as it would have been once both of us die. This has been a bit tricky to organise. If some greedy family member spoils it, it doesn’t matter much, because we each have enough to live on.
We were discussing money the other day, and SO made the point that the way we have always done things has meant that money has never been an issue between us. He is better at paying bills, so he pays most of the bills. We both think the split is fair, and each time it has changed we have worked together for a division that we are both happy with and that we both think is fair. If something starts to irk one of us, we work out a way to make it fair.