How can I utilize different source files when building to different platforms?

I have a project that I am currently building two instances of. One for VR, and one for WebGL. Both builds are essentially the same game, but the WebGL build needs to utilize lower quality models and textures for the sake of file size.

Is there a way to do this dynamically so that I don't have to create two separate projects per platform? My only other idea is to have a duplicate of each scene, one with lower quality assets, and one with higher quality. I feel like Unity has to have an easier way to do this?