Sometimes researchers upload their unpublished manuscripts to pre-print servers so that they can share early results and get feedback from their peers. They will also submit to journals, so many of these articles will undergo peer-review, be published, and appear in databases like the ones listed above.
But not all of them. If you look for articles on a pre-print server, just remember they haven't been peer-reviewed. If you find something interesting, look up the title in one of the databases above to see if the article was ever officially published.