How do websites load data from database on runtime and render blocks of data

Websites like facebook, amazon load some part of data and then when we scroll down further data is getting loaded for same query.So how do they load some part of data from database.How do they receive blocks of data rather than whole in one packet. It doesn't seem like they make any other request for the same search query as data is rendered very fast on scrolling(though not instantly).So how do they do this exactly?