This is a video highlighting the visual effects used in The Mandalorian, specifically how the backgrounds for scenes are projected behind the actors in real time instead of green-screening them in later (and can also be edited and moved on the fly). So the backgrounds you see watching the show are the actual projections displayed behind the actors as it was filmed. I did not know that. Although, like my grandpa always used to say, you could probably fill a book with all the things I don't know. "Just one book, GW?" Not even a whole book really, more like a leaflet.