There are many ways to approach building a search engine for use in eCommerce purposes. In my career, I have created product strategies and worked with development teams to build both heuristic-based and machine learning based search strategies.
For a sample of my approach to a heuristic-based experience, check out Building Heuristics-based eCommerce Search.
For a sample of my approach to a machine learning based search strategy, check out Building Machine Learning based eCommerce Search.