Reactive programming is about conceptualising our application as streams and operations that are performed on those streams.
Observables are a blueprint for creating streams and plumbing them together with operators to create observable chains.
RxJS is a library that lets us create and work with observables.
Angular has a few public facing APIs that use Observables, the EventEmitter, Forms and HTTP.
You are not forced to use observables in Angular, you can use as much RxJS or as little as you want.