'I actually worked in TV before computers. We would make the t | ELI5

"I actually worked in TV before computers. We would make the titles with press-on letters (I forget the brand name) and shoot them with a camera. The title camera would be combined with the live camera in a device called a "luma key" that would switch the live camera off and the title camera on everywhere that the lettering appears, based on the brightness. This was with monochrome cameras. With color cameras there was a device called a "chroma key" that would switch based on hue, usually tuned to blue. The same device was used for example to put graphics behind the weather man. You had to be careful the talent didn't wear any blue clothing."