Doesn’t alter the existing stream. OBS creates new virtual camera that it outputs from your mix of input sources. You then tell zoom to use the OBS virtual camera as its source.

Instead of "altering" a stream, you use OBS to take the camera input, then output it via virtual cam. Once you have the basics setup, you can get fancy setting up scenes, overlays, color corrections etc. Works great for green screens as well.

https://www.nextofwindows.com/how-to-use-obss-virtual-camera

(There are other articles that talk about using a plugin, that isn't required after OBS v25, they integrated the feature).