Open Photo Rendering Settings. Select Engine: Cineware by Maxon then a bit further down select Renderer: Redshift. I would imagine like many of us, unless you have a TOP specification graphics card then you will be politely told you can't run Redshift. I can't help thinking Redshift will be serving a very limited user group for some time. I have a two year old Mac with an 8Gb Radeon Pro 580X graphics card and it didn't make the benchmark.
Apple iMac macOS Big Sur / AC24UKI (most recent builds)
I have a 3080 card and it works without any problems and is fast. However, it may be better than the standard but don't think its all that significant. i think we need some instruction videos on the optimum settings