Most scientific journals are owned by Elsevier and their sub divisions. They can make it very expensive to subscribe to a journal ($500usd or more per year) or buy one study ($20usd and up). So I decided to list some sites for free studies.
https://archive.ph has copies of websites. You might find studies there.
Pubmed. Has 1 million+ free studies. Some studies just have the abstract on the site, for some studies you must go offsite to find the study. Some studies charge for the full study but not the abstract. Registration not required but with free registration you can save studies in folders. https://pubmed.gov
Science Daily has science news and is more likely to have a link to a study.
ScienceDirect. Search for peer-reviewed journals. https://sciencedirect.com
Google Scholar may have studies but expect it to be heavily censored.
JSTORE. https://jstor.org
CORE is a multidisciplinary aggregator of OA research. CORE has the largest collection of OA articles available. It allows users to search more than 219 million OA articles. While most of these link to the full-text article on the original publisher's site, or to a PDF available for download, five million records are hosted directly on CORE. https://core.ac.uk/
Science Open. Registration is required but free. https://www.scienceopen.com/
Directory of Open Access Journals. https://doaj.org/
The Education Resources Information Center (ERIC) of the Institution of Education Sciences allows you to search by topic for material related to the field of education. Links lead to other sites, where you may have to purchase the information, but you can search for full-text articles only. You can also search only peer-reviewed sources. https://eric.ed.gov/
arXive e-print archive, curated by volunteers, run by Cornell university. https://arxiv.org/
Has preprints (studies that are not peer reviewed.) https://preprints.org
SSRN library of 1 million+ studies. https://www.ssrn.com/index.cfm/en/
Elsevier Open Science. https://www.elsevier.com/about/open-science
The Free Library focuses on open science and free access to studies.
Figshare. https://figshare.com/
MedRxiv. May contain some preprints (studies not peer reviewed). https://www.medrxiv.org/
Library Genesis Project (libgen). This has some free studies but it is blocked by many routers and companies. You will have to find out what the address is currently. As of Dec 2022 try: https://libgen.me/
Researchgate. https://researchgate.net
100 search engines for academic studies. https://www.teachthought.com/learning/100-search-engines-for-academic-research/
Lancet. Not all studies are free here. https://www.thelancet.com/
More links
List of free studies. https://www.scribendi.com/academy/articles/free_online_journal_and_research_databases.en.html
More free study sites. https://pastebin.com/2vGVq05X