The structure and kinematics of the gas around and in galaxies are crucial for understanding the multiphase flows within the galactic ecosystem and, thus, galaxy evolution and star formation. Traditionally, insights into the structure of galactic winds and the circumgalactic medium have been investigated through absorption line studies. Recent advancements in instruments and techniques, however, offer a new perspective on gas flows around galaxies through emission lines like Hydrogen Lyman-α (Lyα) and metal resonance lines such as Mg II, C IV, O VI, and N V doublets. Radiative transfer studies are required to decode information because of their resonance nature. During my presentation, I will show results of a 3D Monte-Carlo radiative transfer simulation 'RT-scat.', specifically exploring the radiative transfer of Mg II doublet λλ2795 & 2803 and Lyα, tracing cold gas at T ~ 10^4 K. Apart from showing synthetic spectra and escaping fractions, in particular, I will focus on the powerful observables that have received less attention in the past: their escaping fractions and the Mg II doublet ratio. I will show how these observables can help to constrain cold gas flows. Time provided, I will briefly share the results of interpreting Mg II spectra of star-forming galaxies at z ~ 1, obtained by MUSE/VLT through radiative transfer modeling.