Can somebody explain to me how to change/edit the custom background level? Should I edit the preexisting picture, add my own, or something else entirely? Thanks.
You can edit it with any editing program that supports transparency, such as Paint 3D, a default Windows 10 application. Once you opened the file, you can do whatever you want with it, like paste other pictures over it as long as you don't modify the resolution of the picture and keep transparency. You can use Paint to easily resize any image to a size that fits the custom background, then paste it in Paint 3D. The size of the custom background's upper part is 288x162, which is standard 16:9 format.