These databases are independent of one another and independent companies own these databases. For example, PubMed is an independent database of medical research provided by the National Library of Medicine (NLM). The NLM only files and maintains journal articles about medicine into the PubMed database.
There are other subject-specific databases owned by independent companies:
The journals in these databases are not exclusive to each database. For example, there are journal articles filed in ERIC that are also filed in PsycArticles. But not all of the articles in ERIC are also in PsycArticles.