docs: add working mechanism

docs/faq/how_does_it_work.md

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+# How does Discord Player actually work?
+
+- Discord Player can be used by first initializing `Player` class with your discord.js client. Discord Player uses `Queue` to assign queue manager to individual guild.
+Which means each guild will have its own queue object. Every player action has to go through the `Queue` object for example, `play`, `pause`, `volume` etc.
+
+- When `Player` is initialized, it creates a cache to store external extractors or queues information. Queue is created by calling `createQueue` method of `Player` instance.
+A client should have only one `Player` instance, otherwise it will be complicated to track queues and other metadata.
+
+- Searching tracks can be done via `search` method of `Player` instance. Discord Player offers `search engine` option to target specific searches. Discord Player first
+calls all the registered extractors first with the search query. If all external extractors failed to validate the query, player then passes the query to built-in extractors.
+Invalid or unknown queries may return `arbitrary` result.
+
+- The track result obtained from `search` can be loaded into `Queue` by calling `queue.addTrack`/`queue.addTracks`/`queue.play`.
+
+- Player calls `onBeforeCreateStream` if user has enabled the function while creating queue. This method runs each time before stream is downloaded. Users may use it
+to modify how and which stream will be played.
+
+- Queue is based on FIFO method (First In First Out)
+
+- Final stream is a pcm format, required for volume controls which is created by Discord Player itself.
+
+- All the audio filters are handled by FFmpeg and stream has to reload in order to update filters.