What do writers sell?
Writers sell licenses. That’s it. Or at least that should be it.
Writers OWN copyright. But writers should not be SELLING copyright. Copyright is ownership. It’s the “these words are mine and no, you can’t use them without my permission” bit.
So when writers sign contracts, they aren’t signing away copyright; they’re signing away the license, or right, to use, disseminate, print, or publish those words. The words themselves remain belonging to the writer.
There are a few places (actually, there are many more than a few places, unfortunately) that have awful contracts where the author actually does sign away copyright, giving all the words they’d written to a company. There are even places that demand ownership of your publishing name, disallowing a writer to publish under that particular name anywhere but with that company. [Please don’t sign these.]
The only time you should, as a writer, be selling copyright, is when you are ghostwriting (because during ghostwriting you’re paid a lump sum to writer someone else’s story for them, so they get the copyright of it) or during some form of shared-world ownership situation. (Think DnD, DragonLance, Forgotten Realms, TV show tie-ins, though even then, very often, the copyright remains with the author and they simply have incredibly strict contracts on what they’re allowed to write).
So please check your contracts over carefully! Make sure you’re not giving away something you shouldn’t be, particularly in smaller press or magazines where the publishers might not be fully aware of what they’re asking for.