Google Scholar is able to search 'the deep web' where many of our subscriptions live.
This does not mean everything on Google Scholar is 'free'. It can pull results from our holdings, but unless you are signed into our subscriptions with your EID/password, you cannot access the full articles.
Not everything on Google Scholar should be considered scholarly and there is no way to use the search to limit to scholarly. Like all Google products, the algorithm is proprietary, so I can't tell you how the results are pulled and from where.
What I advise is that you evaluate each source on its own merits. See page at left for help with evaluation or contact a librarian.