Here is an example of how to use upscale models like ESRGAN. Put them in the models/upscale_models folder then use the UpscaleModelLoader node to load them and the ImageUpscaleWithModel node to use them.
Here is an example:
You can load this image in ComfyUI to get the workflow.
If you are looking for upscale models to use you can find some on The Upscale Wiki