Quantum Corrected and Nonsingular Black Holes: Theoretical and Phenomenological Aspects

General Relativity is a cornerstone of modern physics and has now achieved the status of precision science. Despite its success in explaining a wide range of phenomena, significant open questions remain about the fundamental nature of gravity. These unresolved issues suggest the need for a quantum extension of Einstein's theory, which has remained elusive until now. \\ This thesis explores various instances of quantum-gravity effects in different setups, focusing on their impact on the structure of black-hole horizons, spacetime singularities and the observable phenomenology. A quantum-corrected BTZ black hole constructed on a brane in AdS4 spacetime is first examined, providing evidence that these quantum effects strengthen the weak cosmic censorship, thereby enhancing the stability of the black-hole horizon. Subsequently, the spacetime geometry generated by a point-like source in a superposition of different locations is considered. It is shown that the quantum corrections, induced by the position uncertainty, excise the conventional Schwarzschild singularity and generate a regular geometry interpolating between a nonsingular black hole and a traversable wormhole. Furthermore, a broad class of de-Sitter-core nonsingular black-hole models is constructed, which are sourced by an anisotropic fluid, assumed to encode quantum-gravity corrections. It is shown that these solutions have several interesting features: presence of an additional and possibly super-planckian "quantum” hair, a regular, extremal black-hole state and a thermodynamic phase transition favoring black holes with super-planckian hairs. A two-dimensional model of a regular black hole retaining these features is then used to address the information loss issue: the evaporation process at semiclassical level is examined, including backreaction effects. It is found that the radiation entanglement entropy decreases after reaching a maximum and that the endpoint of the evaporation is a regular, zero-entropy state. This suggests a profound (and often overlooked) connection between the resolution of singularities and the information problem. Some new regular solutions of Einstein-scalar gravity in four dimensions are also analyzed, formulating and extending no-go theorems on their existence. Additionally, certain unstable, regular, horizonless scalar solutions are identified that describe the nucleation of a dS4 out of an AdS4 spacetime in the UV regime, with potential implications for characterizing string-theory vacua and cosmological inflation. Finally, on the phenomenological side, a way to constrain the strength of quantum-gravity corrections using data from the S2 star orbit around SgrA* is explored. Additionally, another novel approach to test the regular-black-hole paradigm is proposed, which is based on a nontrivial coupling of nonsingular black holes with the large-scale cosmological dynamics. Although observational evidence for this effect remains contentious and debated, confirmation of such coupling would provide compelling evidence for the nonsingular nature of black holes.