Collective dynamics of heavy ion collisions in ATLAS

The latest measurements of collective behaviour in a variety of collision systems with the ATLAS detector at the LHC, including \mbox{$pp$} collisions at 13 TeV, \mbox{Xe+Xe} collisions at 5.44 TeV, and $\mbox{Pb+Pb}$ collisions at 5.02 TeV, are presented. They include $v_n$-[\mbox{$p_{\mathrm{T}}$}] correlations, which carry important information about the initial-state geometry of the quark-gluon plasma and can shed light on any quadrupole deformation in the Xe nucleus, and measurements of flow decorrelations differential in rapidity, which probe the longitudinal structure of the colliding system. These measurements furthermore provide stringent tests of the theoretical understanding of the initial state in heavy ion collisions.