Its not a filter or layer, its the pixels themselves. Half are normal wide viewing angle pixels, the other half are pixels with a much more narrow viewing angle. When activated they just ... switch off the normal ones.

See for example:

https://gsmarena.com/samsung_galaxy_s26_ultra-review-2939p3....

Or from the official Samsung presentation:

https://youtube.com/shorts/qnUVGPkeCCc