Google's crawlers visits websites, crawls and reads everything and then save it in its database. It checks that website has highly optimized content, its back-links, reads all the elements of a website which are important for SEO. If the website has followed Google's rules and regulations, then Google will improve the rank of a website.
Google rank your websites based on quality backlinks & reputation. If you have backlinks from high authority websites then you can ranked well on Google. Also on-page optimization is also an important factor to get ranked on Google Read out Moz or search engine land for more information.
Perfect off page strategy are Article submission, bookmark, blog posting, forum posting, classified, blog comment, social media optimize. That’s all good way to get more back link and traffic. That makes to your site good rank on search engine.
The website ranked in 1st place is the website that the algorithm has given the best score to when taking into account the 200+ factors. Google is constantly reviewing, adjusting and updating its search results, so a website that is ranked 1st today could potentially not even be on the 1st page next week.
User first, Google gives high priority for the user or audience. If more number of users searching for something then it will become a trend that is only showing in twitter with # (hash) tags. So there will be some exceptional conditional, though user is the origin and destiny for all set of SERP rules and regulations.