The CMS Collaboration has conducted extensive studies of heavy-ion (HI) physics at the LHC, focusing on high-density quantum chromodynamics (QCD), precision quantum electrodynamics (QED), and phenomena beyond the Standard Model (BSM). These studies, spanning LHC Runs 1 (2010–2013) and 2 (2015–2018), have provided key insights into the properties of the quark-gluon plasma (QGP), a deconfined state of quarks and gluons formed in high-energy nucleus-nucleus collisions. The CMS detector, originally designed for proton-proton collisions, has been adapted to study HI collisions, offering detailed investigations of the thermodynamic and transport properties of the QGP. The detector's capabilities have enabled the study of various collision systems, including lead-lead (PbPb), proton-lead (pPb), and proton-proton (pp) collisions, as well as smaller systems like xenon-xenon (XeXe) and proton-proton (pp) collisions. The CMS experiment has also explored novel phenomena, such as the QGP-like effects observed in smaller collision systems and the behavior of heavy-flavor hadrons. The study of the initial state of collisions has provided constraints on nuclear parton distribution functions (nPDFs) and tested the Glauber model and $N_{\text{coll}}$ scaling using electroweak bosons. The bulk properties of the medium produced in HI collisions, including entropy, energy density, and collective behavior, have been analyzed, with findings indicating that the QGP behaves like a nearly perfect fluid. The CMS experiment has also investigated hard probes, such as parton quenching and jet modifications, to understand the properties of the QGP. Studies of smaller collision systems have revealed unexpected collectivity in light- and heavy-flavor hadrons, suggesting that the QGP may exist in smaller systems. The CMS experiment has also explored electroweak sector phenomena and searches for new physics, including QED processes and axion-like particle production. The results of these studies have provided a comprehensive understanding of the QGP and its properties, as well as insights into the behavior of strongly interacting matter at high densities. The CMS experiment continues to explore these topics, aiming to characterize the QGP with unprecedented precision and probe novel fundamental physics phenomena.The CMS Collaboration has conducted extensive studies of heavy-ion (HI) physics at the LHC, focusing on high-density quantum chromodynamics (QCD), precision quantum electrodynamics (QED), and phenomena beyond the Standard Model (BSM). These studies, spanning LHC Runs 1 (2010–2013) and 2 (2015–2018), have provided key insights into the properties of the quark-gluon plasma (QGP), a deconfined state of quarks and gluons formed in high-energy nucleus-nucleus collisions. The CMS detector, originally designed for proton-proton collisions, has been adapted to study HI collisions, offering detailed investigations of the thermodynamic and transport properties of the QGP. The detector's capabilities have enabled the study of various collision systems, including lead-lead (PbPb), proton-lead (pPb), and proton-proton (pp) collisions, as well as smaller systems like xenon-xenon (XeXe) and proton-proton (pp) collisions. The CMS experiment has also explored novel phenomena, such as the QGP-like effects observed in smaller collision systems and the behavior of heavy-flavor hadrons. The study of the initial state of collisions has provided constraints on nuclear parton distribution functions (nPDFs) and tested the Glauber model and $N_{\text{coll}}$ scaling using electroweak bosons. The bulk properties of the medium produced in HI collisions, including entropy, energy density, and collective behavior, have been analyzed, with findings indicating that the QGP behaves like a nearly perfect fluid. The CMS experiment has also investigated hard probes, such as parton quenching and jet modifications, to understand the properties of the QGP. Studies of smaller collision systems have revealed unexpected collectivity in light- and heavy-flavor hadrons, suggesting that the QGP may exist in smaller systems. The CMS experiment has also explored electroweak sector phenomena and searches for new physics, including QED processes and axion-like particle production. The results of these studies have provided a comprehensive understanding of the QGP and its properties, as well as insights into the behavior of strongly interacting matter at high densities. The CMS experiment continues to explore these topics, aiming to characterize the QGP with unprecedented precision and probe novel fundamental physics phenomena.