Google scholar is not a human-curated database but an academic search engine of the Web which narrows the results to "scholarly" ones based on machine automated criteria. For instance, Google Scholar will count citations from online slide sets, reports, undergraduate essays and other sources.
Although Google Scholar does not provide a list of the sources that it is searching to find citations to publications, many research studies have demonstrated that Google Scholar has by far the largest absolute coverage. It is estimated that Google Scholar holds more than 389 million records (Gusenbauer, 2019). Google doesn’t specify how often it is updated but some say it is updated on a monthly basis.
Citations to some specific types of outputs, for example, books, book chapters, working papers are likely to be much better in Google than WoS or Scopus which don’t provide good coverage for those. This may be very important to be aware of for academics working in subject fields in which monographs occupy a prominent place.
The downside is that some voice concerns over its lack of differentiating citations that may not originate from peer-reviewed literature as it picks up broader types of publications. Also, it seems that Google Scholar may not pick up citations from older content or give them less weight than more recent ones.
To date, Google Scholar is by far the most frequently used academic search engine, but it is not the only one. There is Semantic Scholar which is a free tool (built at the Allen Institute for Artificial Intelligence) which applies artificial intelligence to extract the meaning from the scientific literature. Since October 2019, it searches publications from social sciences. Open Alex being the new kid on the block (and whose coverage is broader than Scopus and WoS but narrower than Google Scholar) will launch in September 2022.
Publish or Perish is a free and downloadable software developed by (Professor Anne-Wil Harzing) that retrieves and analyse Google Scholar’s data.