How does digital chroma key composting work?

Approximately 12 years ago, a photographer took a photo of me that was superimposed on a background scene. It was not done digitally. I believe it was done using slides. The photographer showed me various backgrounds and photos he had taken in his studio that appeared to me to have been taken at some other location.

I have several questions.

Is this a common practice and how do I do this?

Is everything done in Photoshop nowdays?

Is there a source for stock backgrounds availabe for purchase, or do photographers take their own background images.

-Jay

Click here to read the answer to this question