To ask you to help me understand marriage & finances.

The starting point for divorce is a 50:50 split of everything, but the result may well favour one over the other, according to need (and maybe history - especially if a short marriage).

But, yes. We opened a joint bank account, and used that as our main (and then only) current account. No discussion on spending required - but then we aren't big spenders, and are fortunate enough to have money left at then end of the month. Savings are all individual accounts, and we spend from whoever has the money available at the time. I "bought" DH's car because he'd paid for a patio to be laid 6 months earlier. Typically, I have more money directed at my savings account, as I'm in a lower tax band!

We kind of did exactly what you are saying. We bought our first house together just before we got married and had a one account mortgage. So we pooled everything and had a spreadsheet showing who spent what, who put what deposit in, etc

Then when we got married we scrapped the spreadsheet and just shared everything

It’s never been an issue for us. If we are spending a lot of money we just check it’s ok, and that there is enough money in the current account to pay the credit card. Otherwise we both just spend what we want when we want

I would argue you have already made yourself more financially vulnerable to him by having a child without being married. Assuming you follow the usual pattern you might have 1-2 more and end up being the default parent or using your money to cover gaps in employment then when things break down you can be on your own, responsible for a family and reliant on payment of child support.
We always had our own accounts, more or less split the bills equally. I spent the rest on the family and he spent his on hobbies and going out.
Whatever you do you should both protect yourself throughout the marriage. You could be the cheater too.

just to note pre-nups have no legal standing in the uk. marriage is a legal contract by which you agree for assets to be shared. if you don’t want to do this maybe marriage isn’t for you?

Everyone is different. I've been with my dh since we were young and didn't really have any assets as such back then. We combine everything and it's all shared. We don't consult each other on what we spend unless it's over a certain amount. It works for us because we have the same outlook to money, spending and saving. If you're on the same page about stuff like that it's easier.

If you're going into a marriage with assets already, then you can obviously protect them through a prenup agreement. Lots of people do that now.

Lots of different ways of doing it. I like shared because it's simple and easy. But appreciate it's not for everyone.

Surely you'd need to consult every time you buy clothes/ book a trip/ get dinner/ get nails done etc.
Nope, we just trust each other not to do anything wildy extravagant. Booking a trip would need consulting but only to ensure the dates worked for the other person who was staying behind for childcare. For a trip together then obviously we'd discuss it anyway to decide together where we were going. Buying clothes, going for dinner all normal expenses that the joint account just covers.

If you have joint finances... how did that happen? Did you one day just throw it all in one big pot or was it more gradual??
Pretty much one day. Got a joint account when we were living together and started putting same amount into it but fairly quickly got bored with figuring out how much was fair and as soon as we were married we were both just putting nearly all our salary into it or into joint savings. Less faff.

Gradual for us- we started off with a joint account for bills, food etc and still had our own accounts. Now after years of marriage we still have our own accounts but everything is mostly joint. We never, ever question each other's spending so you definitely don't need to be running every purchase past the other! You just get used to it and trust each other

What is right is different for each couple. We have everything shared and pooled. Everything we have we have built together from nothing. We were very young when we got together. I think it can be different, especially if marrying a little later with assets you want to protect if the relationship doesn't work out.

Legal differences between marriage & living together

It's not as simple as walking away with 50% after a marriage breakdown. Read the link above which explains how the marriage contract protects you.

We have a joint account for bills & separate accounts for our own spending & savings. We have ring fenced property & assets acquired before marriage so that won't be counted as marital assets.

Do you own another property in your own name and was that bought before marriage? Do you own a property with your partner?

Everyone is different, there is no 'official' way to do things. We have joint current and joint savings but also individual current and savings. Our individual is for buying our own treats/clothes/going out with friends/gifts for one another etc but everything else is split. We earn roughly the same.

The legalities of finances after divorce wouldn't necessarily be straightforward/same for everyone.