How can YouTube and Netflix start playing before the whole video has downloaded?

Streaming works by sending video data in small chunks that the player begins displaying as soon as enough has arrived, rather than waiting for the entire file to download first, and it continuously adjusts video quality up or down to match how fast your connection is delivering those chunks in real time.

The video isn't secretly preloaded onto your computer in advance, and while video is technically made of a sequence of still frames, that fact by itself doesn't explain why playback can start almost instantly — the real explanation is the chunked, adaptive delivery method.

When playback pauses to buffer, it means the next chunk hasn't arrived yet, and when the picture suddenly looks softer or blockier, that's usually the player automatically downshifting to a lower quality level to keep up with a slower connection rather than stopping entirely.