Why returning an observable of observable actually works ?

[raskyer]

Hello community,

I have an async action that does the following :

@Action(MyAction)
myAction(ctx: StateContext, action: MyAction) {
  return this.someAsyncClient.getSomeAsyncResponse().pipe(
    map((someAsyncResponse) => {
      return this.somePrivateMethod(someAsyncResponse, ctx);
    })
  );
}

and the private method which is :

private somePrivateMethod(someAsyncResponse: MyActionResponse, ctx: StateContext) {
  return this.someAsyncClient.transformAsyncResponse(someAsyncResponse).pipe(
    tap(transformedAsyncResponse => ctx.patchState({ transformedAsyncResponse })),
    finalize(() => this.finalize())
  );
}

This code actually works, and if I subscribe to the dispatch of MyAction, the subscription execution is after the end of this.finalize().
But I don't understand why, because what this code make (by doing a map instead of mergeMap) is returning an observable of observable.

Do ngxs subscribe to outer observable that one observable may return ?
Why mergeMap isn't necessary here ?