How do search engines decide on their ranking?

You cannot pay a search engine in return for a high ranking in the natural results. You can get a high ranking if your content is seen as relevant by the search engines.

Search engines identify relevant content for their search results by sending our ‘spiders’
Or ‘robots’ which crawl (analyze) your site and index record its details. Complex algorithms search engine’s search results.
Indian Real Estate Agents

They have what you call search engine algorithms and every search engine have their own unique algorithms using... Those algorithms are kept secret and very confidential on them who knows it.