Here is a link to a fun article How to Add Hollywood Special Effects to Your Videos by J. D. Biersdorfer, May 15, 2019 from The New York Times:
With your smartphone, inexpensive software and a bit of cloth or paper, you can make your own “green screen” movies.
No matter what you call it — chroma key, green screen or blue screen — it’s the film and video technique that gives your local TV weatherperson something in common with the Avengers movies: artificial backgrounds inserted behind the action. You simply record your subject in front of a solid green or blue screen, and then add a touch of software magic to change the background.
Dozens of free or inexpensive apps allow you to use the technique on your own clips. It’s a great way to jazz up your presentations and other videos — or to keep children busy with a weekend project filming their own toys in action scenes. Here’s how to get started.
Step 1: Set Up Your Studio
Step 2: Get Your Software
Step 3: Record the Action
Step 4: Select a New Background