How does Chrome incrementally (lazy) load a page such as a long youtube page?

I'm trying to identify the exact mechanism youtube uses to lazy load a page. For example, I want to scrape all links from a user's page with wget or something similar, but the page is incrementally (lazy) loaded, and does not return all content.

I'm not a web pro, but am a good coder. I have to admit JS is hard to follow in practice. Is this form of lazy loading based in the browser or in the script?