Google Scholar searches cover a range of subject areas; however, Google does not provide details about its specific content covered or proprietary search -- making it difficult to know exactly what is included. It appears that topic coverage is strongest in science and technology with less coverage in the Humanities. So if your topic relates to art, history, or other areas in the Humanities, Google Scholar results may be limited and may not provide the best sources.
Google states that new content is added several times a week, but there may be limitations on newer content. So that you don't miss important sources when you search, it's important to conduct searches in the library databases and other academic or professional resources that are relevant to your discipline or topic. Overall, Google Scholar can be a useful tool as part of your research toolkit, but it does not provide a single comprehensive search.