Post-Processing and Particles rendered into an offscreen framebuffer and ran full-screen shader passes over it by hand. EffectComposer, from three/addons/postprocessing/, packages that pattern for WebGLRenderer: a list of passes, two render targets they ping-pong between, and two dozen ready-made effects. (WebGPURenderer has its own node-based RenderPipeline, which WebGPURenderer touches.)
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EffectComposer
Post-Processing with EffectComposer