This book, the sixth volume in a series on Quark-Gluon Plasma (QGP) research, offers updated reviews on theoretical developments and phenomenological understanding of the hot and dense matter formed in high-energy heavy-ion collisions. Authored by leading experts in the field, it delves into how these advancements help shed light on the recent data emerging from the experiments conducted at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Despite setbacks caused by the COVID-19 pandemic, the book explores a multitude of recent progresses, including insights into jet measurements, study of hydrodynamization in QGP, flow measurements in large and small systems, spin polarization and chiral magnetic effect in relativistic heavy-ion collisions. Additionally, the book features a timely review of the QCD phase diagram in light of the new data collected at the Beam Energy Scan program at RHIC. Furthermore, it includes a chapter on the growing role of machine learning in analyzing and interpretating complex data. Written carefully with detailed analyses and expert reviews, Quark-Gluon Plasma 6 stands as an invaluable reference for individuals engaged in the field.