Massive stars are arguably the most crucial kind of objects in the evolution of the Universe. Although rare---they are outnumbered 1000:1 by Sun-like stars---their radiative contribution dominates the light emitted by galaxies; they are the progenitors of some of the most energetic events in the Universe (supernovae) since the Big Bang, during which myriads of newly-formed chemical elements are ejected into the interstellar medium, and after which neutron stars and black holes are left behind; these compact objects then play a role in shaping the gravitational wave background of the Universe and in the dynamics of stellar clusters and galaxies, possibly fusing into the supermassive black holes that dwell at the centre of most galaxies.

The aim of this thesis is to explore the effects of some physical processes on the evolution of massive stars, as well as the impact of massive stars on the Universe. Massive stars were more ubiquitous in the early Universe than today, as the conditions back then were more favourable to their formation. As such, I have chosen to recount the early history of the Universe, from the Big Bang to the reionisation, in the introduction of this manuscript. This tale is followed by a short history of the scientific discipline of stellar astronomy and a quick introduction to the physics involved in the study of stars. Chapter 2 starts by diving into the physics of stellar evolution in a more technical fashion before introducing the concept of nucleosynthesis: presenting the main processes involved and the various astrophysical sites where they occur. The last section of Chapter 2 deals with the notion of population synthesis, a class of tools I have used extensively during my PhD. Each of the subsequent four chapters presents a published paper of which I am the first author. Chapter 3 introduces a grid of extremely metal-poor stellar evolution models, where the influence of the initial stellar composition was one of the major topics of the study. Chapter 4 presents an article where we examined the impact of the choice of convective criterion on the evolution of massive stars. Chapter 5 presents an article in which we investigated the radiative effect of population III stars (the first stellar generation of the Universe) on the reionisation, in the case where they would have undergone chemically homogeneous evolution. Chapter 6 presents an article where we explained recent JWST measurements of very high nitrogen abundances in galaxies of the early Universe using numerical models of massive rotating stars. The work presented in chapter 7 has not been published yet. In it, we explored the parameter space of hierarchical triple black hole systems, to determine the regions where the von Zeipel-Lidov-Kozai mechanism can induce the inner binary black holes to merge. Finally, chapter 8 concludes this thesis by summarising the previous five chapters and providing perspectives for future work on the topics they tackle.