How to record a live video of yourself with a transparent background?
I really want to know how this gets automatically edited.


The technique is called chroma key and consists of considering as transparent all pixels of the colors within a specified range. This is usually used with a bluescreen or a green background. The image is then simply layered over another background.

As you have noticed, chroma key is not limited to postprocessing: a lot of software products can do this in real-time for live streaming.

By the way, if you have used Google Hangouts, one of the effects consists of replacing the background by an image. Don't be mislead by this, since this has nothing to do with a bluescreen and doesn't require any specific color for the background; I imagine that Hangouts' algorithm rather assumes that parts that don't move are the actual background.