Just then, people start cheering and applauding as Steve steps up on the karaoke stage and takes the microphone.
Written like this, does it mean that people started cheering and applauding because Steve got up on the stage?
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Here, "as" means at the same time. Unless there was some context to doubt it I'd assume that people at a karaoke night will clap each new participant. They aren't clapping because Steve got on stage. They are clapping because someone got on stage.
But the text doesn't say any of that. It only says the clapping was at the same time as Steve getting on stage. Everything else is inferred.
This demonstrates the difference between direct meaning, and what can be inferred from context and our common sense. We use lots of clues and context to understand the meaning of any sentence. You don't need to tell everything to your reader, you only need to give them enough to fill in the gaps from their understanding of the world.