Afaik dlss and fsr are pretrained models and they don’t train them for each specific game.

Maybe I misunderstood it and its not about adjustment training to specific games, and its about adjusting settings or something like that. I searched for a resource to point to that, but cannot find. It's really hard to search about this topic online, because its always the same basic explanations what Ai upsaclers are (probably written by an Ai model itself...).

Modders totally do enable them for games that don’t support them natively

I know of modders being able to replace an existing model that already has motion vectors and depth. It would just reuse those information, because the API is agnostic to the models. If modders add vectors and depth by their own, that does not change the fact that DLSS or FSR 2+ can only work with those data.

I don't know the exact process too, so we are both speculating. But its fact that Temporal upscaling (that is FSR2 and upwards for FSR related ones) need integration from the game. And these are Ai models that needs some kind of training data from the game as well. I don't know what that exactly means, but my guess is that the integrated Ai model to upscale is trained with the data in the game and then delivered as a package. That makes it impossible to integrate a general purpose Temporal upscaler with Ai into a system, that could be utlized on any game (or program if you will). So I don't know how this cannot be anything else "out of question". I mean this is by concept not possible, as far as I understand (and why nobody did it too).

But an Ai system could be used as a Spatial upscaler, like a filter that is not integrated into the game. FSR 1 itself is not using any Ai, but DLSS 5 has such capabilities (think of the new lighting model, it could be tweaked to upscale the image instead).

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?