That's only the case for so-called Gold Open Access artiles. However, many other articles are put behind a paywall, but also have a version uploaded elsewhere (

That's only the case for so-called Gold Open Access artiles. However, many other articles are put behind a paywall, but also have a version uploaded elsewhere (e.g. arXiv) by their author ("Green Open Access") - Unpaywall helps you find those versions when you hit a paywall.