My understanding as a non-lawyer is that unless there's an assignment of copyright by contributors they retain copyright to their contributions. I think in practice they also license that contribution under the project license, but some quick duck-duck-go research indicates that this is at least a complex topic. The most reliable source I was able to find in the time I care to spend on it is from the FSF[0].
Even in the case where copyright ownership is assigned to the project owner, it is a very different situation than the case where your created content is just hosted on a service that claims rights to it.
Even in the case where copyright ownership is assigned to the project owner, it is a very different situation than the case where your created content is just hosted on a service that claims rights to it.
0: https://www.gnu.org/licenses/gpl-faq.en.html#AssignCopyright