What is the difference between flatMap, concatMap, and switchMap?
flatMap: Subscribes to inner publishers eagerly and flattens them. It does not preserve the original order of elements (they are interleaved/asynchronous).
concatMap: Similar to flatMap but waits for each inner publisher to complete before starting the next one. It preserves order but is slower because it is sequential.
switchMap: Every time a new element arrives from the source, it cancels the previous inner subscription and starts a new one. Ideal for search-as-you-type scenarios.
