Making new demos and making demos play when the game starts

Is there a way to do this: When you start the game, it'll show a demo and you have to click to get the menu open and when you start a level the demo stops? Also how do you make your own demos?

Wow, this question brings back memories. I'm pretty sure recording demos was the second code the community ever learned (the first being "$testcheats=1;"). In any case, type "$dorecorddemo=1;" into the console, and the next level you play will record a demo. Please note that they aren't cross-platform, so Mac demos won't work properly on Windows, and vice versa.

For your other question: clarification. Do you want the demo to keep playing in the background while the menus are open?

Until you select a level, a demo will play. When you start the game, the demo will start.