How do ReadableStream, WritableStream, and TransformStream work — what is backpressure, and how would you stream-process a 500 MB file or a streaming fetch response without exhausting memory?

Imagine you have a big bucket of water (data) and a small cup (memory). You can't pour all the water at once or it will spill. So you use a pipe with a valve: the pipe lets water flow cup by cup, and the valve tells the bucket to wait when the cup is full. That's backpressure — it stops the flow until there's room.