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).
There are some non-divestable rights out there. Canada (and others) have a copyright concept of moral rights that cannot be given away by contract or, in other words, nobody can ever force someone to give them away. An artist/creator can decide to not exercise them but the artist/creator retains them regardless of contract language.
>> Unlike other IP rights, moral rights cannot be sold or given away. Even in the case of a sale, an author retains their moral rights in the work, unless they choose to waive these rights.
https://ised-isde.canada.ca/site/canadian-intellectual-prope...
I was thinking about this as they were covering up murals and stadium names for the world cup. Canada doesnt really do that, but canadian stadiums are not generally named after tech companies (ie BC Place got to keep its name).