How does a website use another website's data for their own?

Question

For example:
YouTube live subscribers counter websites _ how do they get the data / info from YouTube so they can use it on their website?
There's probably PHP involved in updating live sub count,
...
I'm just wondering how they do it.

Answer 1

The client website is fetching data from API of data website
Take for example, this is fetching HTML source code.
https://code.sololearn.com/WO2Pg5Gb9T56/?ref=app
For data from RESTful API, it is usually of JSON format.
Stay tuned with the async series
https://code.sololearn.com/WKKkpq0efxai/?ref=app