Does SE return the copyright to its contributors in order for them to partially/completely republish their own material in some peer-reviewed journals, or any written permissions from SE are required?
As mentioned in the comments, the Stack Exchange terms of service speficy that:
- You retain the copyright of any content you post to Stack Exchange
- When you post, you are making the content available to the public (and within that, to Stack Exchange) under a Creative Commons Attribution-ShareAlike license (with the precise version depending on when the content was posted).
As far as Stack Exchange is concerned, you own the content and you are able to publish it in any form you want.
Of course, pre-publication on Stack Exchange could make some journals and publishers refuse to publish the material, but that is a question that we cannot answer for you.
While you mention publishing your own material, note that if you are publishing material from answers or questions by other people you need to at least make that clear.
There have been cases of people selling eBooks based entirely on Q&A material from sites on SE. That was material they did not themselves write. This is seemingly legal. You would, however, normally be required to acknowledge the use of material from SE in any such publication (as was done in those cases eventually, IIRC). You should not e.g. publish material from an answer by someone else and not make it clear that it was from them.
I would recommend reading the posts about the subject on this Q&A : Somebody scraped our answers and sold them as a book
Does SE return the copyright to its contributors
No, SE can’t “return” something that they never had. Each contributor owns the copyright on their contributions the whole time (assuming these contributions are actually their own work), unless they enter into some other agreement regarding this copyright.
Contrary to what is suggested in the top-voted answer, I cannot find a clear statement of this. But we don’t need one. Copyright doesn’t get transferred unless applicable law specifically provides for this or the copyright owner agrees, and neither of those applies to SE contributions in general.
in order for them to partially/completely republish their own material in some peer-reviewed journals
Even if SE owned the copyright and subsequently transferred this ownership back to the contributor, the contributor would be free to exercise all of their rights under copyright, not just publishing the material in selected publications.
You may be thinking of how some journals require authors to transfer the copyright to the publisher. In that case, the publisher may licence certain rights back to the authors. That is different to transferring ownership back to the authors, which is what the phrase “return the copyright” suggests.