The scene is rendered into two canvases, one with the full scene and another one with just the glowing objects. The 'glow-canvas' has a css blur filter. Overlapping the two canvases works similar to blending them, so the final scene shows the blur as a glow.
The sphere at the center works as a Point Light making the spheres look as they are actually receiving light from the glow.
You can use the [rustic] camera controls to see the real-time rendering of the glow.