The study of the Quantum Chromodynamics (QCD) phase diagram has been the object of great effort in the scientific community both from theory and experiment, and has been investigated from first principles through lattice simulations in the low density regime.

The transition between ordinary matter and a deconfined Quark Gluon Plasma was shown from lattice QCD to be a smooth crossover. It is probed experimentally in relativistic heavy-ion collisions at the Brookhaven National Laboratiory (BNL) and CERN, where sufficiently high temperatures are reached to create such a system. This transition is believed to become of the first order at large baryon density, implying the existence of a critical point. The search for such a point is the main goal of the second Beam Energy Scan (BES-II) program at BNL.

In this dissertation, I focus on the transition region of the QCD phase diagram by calculating several observables with different theoretical approaches. In the first part of this work, I study the moment in heavy-ion collisions at which the chemical composition of the system is fixed: the chemical freeze-out. I compare net-particle fluctuations calculated in the Hadron Resonance Gas (HRG) model to experimental data to determine the chemical freeze-out temperature, with particular attention on the dependence of such temperature on the flavor composition of the particles. A separate analysis is performed to study the content of the hadron spectrum, through a comparison of HRG model and lattice QCD results.

I also generate a family of Equations of State (EoS) for QCD matching lattice calculations at low baryon density, and including a critical point in the correct universality class. Moreover, I investigate the consequences of the critical point on measurable quantities, providing guidance for the forthcoming data from the BES.

Finally, in light of the recent extension of hydrodynamic simulations to include all conserved charges of strong interactions, I produce an EoS for QCD that depends on all three chemical potentials. I then study the impact of these additional conserved charges on the thermodynamics when realistic conditions on the composition of the system are imposed.