[MrDOS]

No, contributors to FOSS generally do not give away their rights. They contribute to the project with the expectation that their contributions will be distributed under its license, yes, but individual contributors still hold copyright over their contributions. That's why relicensing an existing FOSS project is such a headache (widely held to require every major contributor to sign off on it), and why many major corporate-backed “FOSS” projects require contributors to sign a “contributor license agreement” (CLA) which typically reassigns copyright to the corporate project owner so they can rugpull the license whenever they want.

Stealing from FOSS is awful, because it completely violates the social contract under which that code was shared.

[yeahforsureman]

You're still mixing up contributor license agreements with the kind of arrangements where the copyright is actually transferred and assigned "away" from the creator to another copyright holder (generally a copyright assignment agreement). This is far less common than CLAs.

I don't know what you mean by a rugpull exactly, but of course in theory you can grant/obtain very extensive rights under a CLA as well, including eg the permission to relicense your contributions under whatever terms the licensee prefers. CLAs are a great way to centralize the IPR in an open source project for practical purposes like license enforcement, but in case the CLA terms allow it, the central governing entity could also obtain the right to switch the license even to a, say, commercial one. (Such terms would usually be a red flag for contributors though.) And in any case, that kind of CLA wouldn't still close off the code already released under the previous open-source license, and neither would it prevent you from licensing your own contributions under terms of your choice.