How to implement long scroll behavior?

So I am creating a website that in the end will have long scroll behavior. I have seen a website that is a very good use of such behavior and have been trying to figure out how it works.

Here is the link: http://le-mugs.com/

I have also seen another example although I'm not sure whether it's a long scroll behavior example or not but I also couldn't figure out how it works.

Here's the link: https://www.booklocal.in/

I would be grateful if somebody could name the reliable resources form which I can learn how to implement this behavior or maybe some library. Thanks in advance.