There is like 50 page agreement, you even have to give up your copyright rights! The only way to do it legally in my country, is to hire editor as an employee!!! (Contractors can not legally give up copyright to their work)
Have you ever read the ToS/ToU of any social media site? Did you know that by using this site you've agreed to arbitration? https://www.ycombinator.com/legal/#tou
Giving up copyright when you write an article for Wikipedia is literally the only way it could possibly work. The biggest issue Wikimedia has is its full time staff, followed by full time editors.
You license your contributions under an open licence. You don't give up your copyright. There would be no other sensible way to operate a collaborative encyclopedia without a license of this kind.
I (lawyer) have never encountered a jurisdiction where a contractor could not license their work under the contract with their employer (the person contracting them).