API is like an easy gateway sites offer to make sure other developers can integrate data from themselves to their apps. Reddit used to be more permissive towards third party apps back when they didn’t have an official one; but now that their data is used for other purposes and they have an official app; they didn’t want to give out their data for free anymore.

All fediverse sites use an API called “ActivityPub” to have their content work with each other, so a lemmy account from lemmy.ml can follow another user from lemmy.world and their posts can be seen on a third party app.

NewPipe uses data scraping to get the data from youtube from the data the site gives. This is not using an API, it instead acts like a user and gets the data from the site itself. Sites can introduce various methods to distrupt data scraping; but cannot truly do it without crippling the anonymous user experience a lot. Lots of modern media sites have started to do so you might have realised.