how to download videos from websites with java [closed]

I'm trying to create a software which can download video from any website, but I don't know how to do that. Some websites just place a video element with the direct link to video but others not.

I want to know how can i able to download videos from that websites (to be precise assume sites like Youtube, Vimeo and Facebook)

I googled it and get many solution but the other answers are very straight forward without any explanation. So,

I want an Explanation of how that can be achieved. I am not aiming toward any API. I'm doing this as a learning Project so I willing to know the entire thing.

Please Explain any code you give