Google Scholar has more unique types of materials (PDF files, Word docs, technical reports, theses and dissertations, etc.). Scopus has “some” proceedings and books but mainly covering journal articles.
The h index of any individual is correlated to the number of documents used to calculate it.
Web of knowledge has the fewest number of documents referenced, thus h index is smaller with WOK. Then it is SCOPUS. Research gate give a much higher h index. BUT the highest h index for an individual is with Google Scholar.
It is easy to retrieve from the software HARDINGS "publish or perish". It is a free software quick to install and easy to use.