These e-journal collections provide access to many journals in the field of History, but they are more limited in coverage compared to subject databases. In most cases, it's better to search subject databases to identify articles, and then use the "check for full text" option to link to the materials in these e-journal collections.
There are several databases from which to choose when you are seeking scholarly work. Most databases for historians focus on a particular geographical and/or chronological period. Always make sure that you've selected the most appropriate database to search!
Start with the title of a book recommended to you in a subject encyclopedia article. Once you've found that book in Primo, click on "item details" to see what the Library of Congress Subject Headings are. You can click on any of those to find more books that share that subject heading!