Khan academy is a big platform for free online leaning website https://khanacademy.org where we learn lots of subjects topic like mathematics, biology, economic, physics, chemistry, history and diffrent types of topoc . co-founder of khan academy salman khan.
This organization produces shorts lecture in the form of youtube video . khan academy app you get at playstore or app store in application you get all subject to related you problem if you want offline video download video free.