I’m not familiar with how they do it in particular, but I do have some experience with what AI art in general can do.
I know there’s techniques to generate larger images, but it takes more computing power so I doubt you’d find a place doing it for free. Your simplest method would be to use a simpler AI upscaler like waifu2x. It won’t be able to add meaningful details, but it can still make it look okay at a higher resolution. Beyond that, you could start with a smaller image and then outpaint to add more to it. Or maybe generate on image, then upscale parts of it and using upscaling and outpainting from the parts already generated to do the rest.
I’m not familiar with how they do it in particular, but I do have some experience with what AI art in general can do.
I know there’s techniques to generate larger images, but it takes more computing power so I doubt you’d find a place doing it for free. Your simplest method would be to use a simpler AI upscaler like waifu2x. It won’t be able to add meaningful details, but it can still make it look okay at a higher resolution. Beyond that, you could start with a smaller image and then outpaint to add more to it. Or maybe generate on image, then upscale parts of it and using upscaling and outpainting from the parts already generated to do the rest.