Well, it's less common for fiction but if you're working for a company and writing content you're almost certainly giving up any rights.

If you're ghostwriting without credit, it's not explicitly spelled out but the way it works is they'll slap it on their copyrighted website, possibly under someone else's byline. And they'd reasonably expect you not to publish the same thing verbatim somewhere else.

Though you also have cases where cross-posting is at least tolerated.