Abstract: The transition of strong-interaction matter from the hadronic phase to the quark-gluon plasma phase is a rapid crossover but not a true phase transition in nature. The true phase transition of strong-interaction matter is expected to exist only in certain limits, e.g. chiral limit of massless quarks and etc. In this contribution to CNPC2023 Special Issue we present our recent studies on the true phase transition of strong-interaction matter in the chiral limit of massless quarks as well as its microscopic origin. The study is based on (2+1)-flavor lattice QCD simulations using highly improved staggered fermions, with pion masses ranging from 160 MeV down to 55 MeV. Utilizing a newly proposed method to compute the quark mass derivatives of the Dirac eigenvalue spectrum on the lattice, it is found that the axial U(1) anomaly is still manifested at 1.6 T_\rm c, with a microscopic origin consistent with the dilute instanton gas approximation. Furthermore, based on lattice QCD results and a generalized Banks-Casher relation, it is found that the macroscopic singularity of the chiral phase transition is encoded in the correlation of the Dirac eigenvalue spectrum. Future research directions along these findings are also discussed, including the investigation of the temperature range between T_\rm c and 1.6 T_\rm c to understand the breakdown of the dilute instanton gas approximation and its connection to the chiral phase transition.