Look into parallax background. This will give you an infinite background.
https://gdscript.com/solutions/godot-parallax-background/
To make the map infinite takes more work. You can use TileMaps. This is like the kind of levels from the original Mario game. You can create 2 TileMaps next to each other, and when the player gets to a certain point, you move the one in the far side to the near side (adjacent to the one on the screen). This gives you a fake infinite map. Or you could do things totally random or procedural.