How do locally generated files work and how do I implement them on server and client side?

What I mean by "locally generated files" is when you download a file from Google Drive it is fast, really fast on bad networks.

But on other sites (like downloading Windows from Microsoft) it utilizes the network to the fullest.

To give one more example, look at fastest fish. Network files download use the network to their fullest.

But what is puzzling is how fast the same file using locally generated option is. It actually finished downloading while I was editing this snip.

Like how cool is that downloading 1 GB in less than 10 seconds?

To quote them

These locally generated files will download much quicker than the network downloaded files since their content's are generated on the fly. Their download speed is restricted only by your device's processor and storage drive speed.

What is puzzling for me is where do they get the extra information on bad networks, like where does it get more information from a 200kbit stream?

• It isn't file compression
• You can't hardcode it in the HTML as it won't load the page from the servers

So the main question:

How do I implement this cool feature in a Node.js server and serve it at high speed in bad network connections?