What exactly do you mean? Any link? As explained. FSR 2, 3 and 4 needs training data from the game. It is possible to swap DLSS out with FSR variants, because DLSS has the training data already. But you cannot have FSR 2, 3 and 4 on all games like FSR 1. And FSR 4 also has to modes as it seems, one classical Temporal like 2 and 3 that runs on relatively older hardware too, and the new ML-based variant that requires the new cores on RX 9000 hardware.
If you use Optiscaler, then I think what happens is that you swap the DLSS implementation with FSR 4 Temporal variant out. I searched the web a bit and found this: https://github.com/optiscaler/OptiScaler
OptiScaler is a tool that lets you replace upscalers in games that already support DLSS2+ / FSR2+ / XeSS
Enables usage of XeSS, FSR2, FSR3, FSR4 2 (officially, RDNA4 and RDNA3 dGPUs only) and DLSS in (temporal) upscaler-enabled games
So what part I was wrong about exactly?