Banding the sides of an image is called pillarboxing (as opposed to banding the top and bottom, i.e., letterboxing). Wikipedia's page refers to the technique you describe as "stylized pillarboxing" or "echo pillarboxing".
Yes. Give each layer a layer mask. Where the layer mask is white, the layer shows. Where the layer mask is black, it is invisible. Gray values between black and white make the layer blend with that opacity.
So make each layer mask the appropriate triangle of black vs. white and you'll be set.
Try adding some noise to the sky. This will ease the transitions between the bands of color. Here's a good reference for doing so: https://www.dpmag.com/how-to/tip-of-the-week/identifying-repairing-banding/
For MacOS, there is an app, StarryLandscapeStacker that helps automating the process of creating these types of photos, where you have both landscape, and a starry sky.
Look at the web site - there is a lot of information about how to shoot and process; what raw conversion options to choose, etc.
This and This are both images I have created using this ...