JSTOR is a digital library, entirely meant to contain back-issues of academic research. Some of their library is open-access, some requires a subscription (and they have a free subscription option).
ProQuest is an information services company. In academic circles, they specialize in publishing and curating libraries of theses and dissertations. Access is dependent on the institutions involved; sometimes you pay ProQuest, sometimes the Universities will pay ProQuest to keep their thesis/dissertation library open access. Essentially every university that uses ProQuest enforces publication of theses and dissertations through ProQuest; you literally have to publish the documents there in order to graduate. They also do other services related to maintaining archival records.
Elseveir is an academic publisher. They run a large number of academic journals, some of which are subscription-based, and some of which are open-access.
Edit: Assuming your question is more about how to find research articles; the best way is to do it in two phases;
1) Use Google Scholar to actually find web links to the papers, which will give you all the information you need about authors and the publishers, and;
2) Use the information in 1 to figure out how to actually get a copy of the paper. Sometimes JSTOR will have it, sometimes you can get the papers implicitly through the thesis/dissertation of one of the authors (as most research at universities is actually done with a grad student as the first author), or you’ll have to either find pay for a copy or email the authors for the paper.
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