You automatically hold copyright over any intellectual property that you create. You don't have to even write a little "c" with a circle around it next to your name on the paper, it belongs to you.
In the US, copyright protects fixed expressions of ideas. The author retains the sole right to do the following with their work:
In the United States, copyright licenses last for the remaining lifetime of the creator plus 70 years.
When an author submits an article to a journal, they typically sign their copyright over to the journal, so then the journal owns the article, and can do anything they like, such as require payment in order for readers to view the article.
There are loopholes where the author can retain the rights over early drafts of the published article. These are the pre-prints that many universities archive in their institutional repositories.
Read more about US Copyright on this US Copyright Office website.