What ML/DL techniques power Youtube/Netflix search systems? Video platforms like YouTube, Netflix, Amazon prime have an excellent search system - given a search string, find most relevant videos. Which Machine Learning /Deep Learning techniques used for this?

Any pointers will be of great help
At Netflix, the machine ​learning algorithms used are more complex since the data is really vast and increases day to day. Take a look at these:

Netflix Recommendations: Beyond the 5 stars (Part 1)

Machine Learning Is Everywhere: Netflix, Personalized Medicine, and Fraud Prevention | Udacity