Mostly by not being sent
How Does a Picture Get to Your Television?
A moving picture is far too much to send, so almost all of it is thrown away first.
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Step 1 of 5
The picture starts as a list of numbers.
A camera measures the brightness of a red, a green and a blue light at every point, and writes them down. What that list becomes at the far end is the screen's business, not the signal's.
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Step 2 of 5
There are far too many of them to send.
A single second of sharp video is about a million times more than a second of speech. Radio's trick of wobbling a wave in time with the sound cannot carry anything like that much.
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Step 3 of 5
So only the parts that changed are sent.
Most of a television picture is not being sent to you. It is already there. Most of a picture is the same as the picture before it. The wall behind a newsreader is sent once, and after that the signal carries little except the newsreader.
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Step 4 of 5
And what is sent is deliberately approximate.
Detail your eye would not have noticed is dropped on purpose. This is a choice about how much to throw away, not a flaw, and you can watch the choice being made badly.
A crowd shot is the hard case: thousands of tiny things moving at once means almost nothing stayed the same, so the picture goes mushy exactly where the movement is.
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Step 5 of 5
Broadcast sends one copy to everybody.
A transmitter sends the same signal whether one set is on or a million are. That is why it costs no more to have more viewers, and why you cannot pause it. Streaming sends you your own copy, which is why you can.
The short version
A picture is turned into numbers, then cut down to the few that changed and the few your eye would miss — and broadcast sends that one signal to everyone at once, while streaming sends you a copy of your own.
Try it yourself
Watch a football crowd on a weak signal. The pitch stays sharp and the crowd turns to mush, because the crowd is the part where nothing stayed the same.