Blender...I did some green screen work in Blender back in 2009 and it was pretty straightforward even back then.

If you just want to play around to get the hang of it without spending anything, last I checked you can get free-to-use, pre-made videos featuring green screen or other chromakey elements and load them up in Blender's video editor. BTW you can use any color you want, so if all you have is a blue bed sheet, you can make it work.

It's not what they're using, but Davinci Resolve also does chroma-key, as well as pretty much anything else in v16. It's also free!

Blender, I suppose, just as the article says.