RxJS Observable-switch && Switchmap

2. "Switch" subscribes to the Observable that emits observables, also known as a Higher-order Observable.
   Explanation: "That emits observables" as a modifier clause in front of "an Observable".

4. Each time it observes one of these emitted inner observables,
   The output Observable subscribes to the inner Observable and begins emitting the items emitted by.
   Explanation: "The output Observable subscribes to the inner Observable and begins emitting the items emitted by that." The spelling of the completion should be:
   "The output Observable subscribes to the inner Observable and (the output) begins emitting the items emitted by (the inner Observable). "That" is "the inner Observable".

6. Returns an Observable, emits items based on applying a function so you supply to each item emitted by the source Obs Ervable, where the function returns an (so-called "inner") Observable.
   In three stages:
   First stage: Returns an Observable that (1. Blah, blah, blah ...);
   Explanation: Return a observable
   Phase II: Returns an Observable that emits items (2. Blah, blah, blah ...) emitted by the source Observable
   Explanation: Returns a observable that will emits items.
   These items are emited from the source Observable.
   (description is two-layer Observable, i.e. Higher-order Observable)
   Stage Three: (2. Blah, blah, blah ...) the segment referred to in the paragraph: based on applying a function so you supply to each item
   Explanation: Each item in the second stage is processed by the function you provided (custom)

RxJS Observable-switch && Switchmap