When you produce work it is therefore legally your own. If you work for a publication such as The Guardian and produce work for them it therefore legally their work.
Things you need to know as a freelancer
things you’ve written you own
content is syndicated online therefore it is more tricky to get copyright
sell your piece to an editor or outright, selling it fully to you as an outright editor
you should maintain your copyright
if you find your work has been used without your permission, it is hard to do something about it, it can be difficult to prove that it is legally your own.