This is very similar to my v3.0 SDXL Workflow except that it doesn't generate a new image from scratch. This workflow will take an existing image that you copy/load in and upscale and refine it. I use this frequently to improve old images I had forgotten about and never published.

Most of the images in this workflow's gallery are old Flux images that were dated and lacked detail. I used this workflow to spruce them up and add the additional detail that only SDXL can provide.

For photorealistic results, I would suggest using either MoP (Mixture of Perverts) or GonzaLomo v4.0. Either will do a very good job of adding detail. The only setting you may want to tinker with is the sampler denoise. A higher denoise will add more detail but might also change the image more than you'd like. I find between 0.15 and 0.30 is ideal.