Videos on TVs and Projectors

I’ve seen videos played on TVs and projectors in Gmod, and other source games. I thought they were just edited in, but they match the screen size perfectly. How do they do it? Some of it, or all, maybe fancy editing, I don’t know.

Machinima?

Greenscreen and/or video editing.

In-Game?

Mod/Addon.

Figured. Thanks.

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