Massive stars are primary sources of chemical yields and mechanical and ionization luminosity budgets. Therefore, understanding massive stars' evolution is crucial to have a complete understanding of the chemical and ionization evolution of galaxies. Moreover, with the advent of instruments like MUSE, JWST (upcoming), HST, etc., we have (/are going to have) a comprehensive picture of the impact of dust-masked massive stars in the evolution of galaxies for the wide range of wavebands starting from IR to optical to UV. The evolutionary paths taken by very massive stars, M > 60MSun, remain substantially uncertain: they begin their lives as main sequence O stars, but, depending on their masses, rotation rates, and metallicities, can then pass through a wide range of evolutionary states, yielding an equally broad set of possible surface compositions and spectral classifications. The surface enrichment of He and N is quite common in rotating WNL stars, but the WNL-like surface elemental abundances in slow rotators, as observed by Herrero et al. 2000, Vink et al. 2017, etc., puzzled astronomers for almost two decades. Meynet & Maeder (2000) hypothesized that an exotic scenario of stellar spin-down needs to be invoked in order to explain the origin of these unusually high surface enriched slow rotators. Contrary to this hypothesis, I discovered that these nonrotating metal-rich stars reveal the products of nucleosynthesis on their surfaces because even modest amounts of mass loss expose their “fossil”-convective cores: regions that are no longer convective, but which were part of the convective core at an early stage in the star's evolution. This mechanism provides a natural explanation for the origin of metal-rich ([Fe/H] ≥ -1.0) slowly-rotating WNL stars without any need for exotic spin-downs. These stars have a huge impact on determining the chemical evolution of galaxies at high redshift. I will also discuss the impact of these stars on determining the ionization budgets in different wavebands.