Here is what the about page says:
Google Scholar aims to rank documents the way researchers do, weighing the full text of each document, where it was published, who it was written by, as well as how often and how recently it has been cited in other scholarly literature.
Here is an academic article that attempted to deconstruct the ranking algorithm: Beel et al 2009, PDF
They suggest that two key factors were the citation count (more citations pushes the article higher up the search list) and the search term appearing in the title. They review a wide range of features.
Note that the article is 10 years old and the algorithm has presumably been updated over the years.
- Beel, J., & Gipp, B. (2009, July). Google Scholar’s ranking algorithm: an introductory overview. In Proceedings of the 12th international conference on scientometrics and informetrics (ISSI’09) (Vol. 1, pp. 230-241).