In the era of JWST and VLT, interpreting galaxy observables requires moving beyond simple kinematic models to account for the complex message encoded by radiative transfer (RT). This talk explores the critical role of scattering processes in two vastly different environments: the diffuse Circumgalactic Medium (CGM) around galaxies and the compact, enigmatic Little Red Dots (LRDs) at high redshift. Interactions with the gas in the galaxy outskirts (i.e., the CGM) and the co-evolution of supermassive black holes and galaxies through LRDs are important for understanding galaxy evolution. I study scattering-driven line formation using 3D Monte Carlo radiative transfer codes, RT-scat and STaRS.In the first half, I will discuss multiphase CGM through hydrogen Lyα and metal resonance doublets such as C IV λλ 1548, 1551 and Mg II 2796, 2803. I demonstrate how these lines trace physical properties of the circumgalactic medium around quasars and star-forming galaxies. In the second half, I will shift to Little Red Dots at z > 5 discovered by JWST. Their broad and asymmetric hydrogen Balmer lines are often interpreted as signatures of active galactic nuclei, implying high black hole-to-stellar mass ratios. I show that resonance, Thomson, and Raman scattering in dense gas can instead reproduce these features without requiring a massive central engine. These results emphasize that, whether in the diffuse CGM or in the dense environments of LRDs, scattering effects on line formation are often the most important factor in interpreting the message of modern observations.
