I cannot say how many percent this are doing. I recognized that most of my photos were not found via keywords. At least SS only seldom displays the keyword matching percentage next to my downloads.
Here is an article explicitly explaining how to search for stock on Google: https://www.stockphotosecrets.com/questions-answers/google-images-search-to-find-stock-photos.html
Looking through some videos on "How to" find Stock Photos Google is mostly the general advice. Google "how do people find stock photos" and look through the youtube videos.
Google even seems to have found that out themselves and plan to adapt their image search: https://www.freepik.com/blog/google-implement-stock-filter-image-search-results/