how to find out the api's of a web app

I am new to web development. My question is as simple as the title is.

How to find out the api's of a website? For example how do I know which are the api's for fetching/posting or doing some stuff on google facebook, youtube etc.

Also how to know what I can do with that api. For example can I only do a fetch request or post request etc or both of them.

Thanks in advance for help.