system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does trade work between the 28 members of the European Union internally and (more importantly) as a group with the rest of the world?

Who contributes which goods/services and how much? Who makes proposals and agreements and who can shoot them down? how is the income divided? How are resources pooled between the 28 countries? Would a war between one of the 28 and another nation (say spain/argentina) effect how the other 27 trade with either?
gpt
Trade for the most part is not between nations but between companies and people.

Governments don't really get all that much involved much at all on trade inside the EU anymore than US states would be involved in intrastate commerce in the US.

The EU as a whole negotiates deals with other countries or unions of countries. for the most part those aren't deals like selling a certain quantity of some product, but setting down the rules by which corporations and people in the countries can trade with each other.

Agreements are between the EU and foreign countries are negotiated as a whole because there is strength in numbers.

The EU collects due from its members and uses that money for various projects in the EU. The poorer countries in the EU mostly get more money invested into them locally by the EU than they pay into it.

As for defence, the majority of EU nations are also in NATO and the EU treaty itself has mutual defence clause in Article 42.7. If a member country gets attacked and invokes this article the rest of the EU would support them. A war that was waged offensively by an EU member would more ambiguous but also far less likely.

There are currently movements in progress to further unite and integrate the militaries of the countries with one another.