The top quark, the heaviest elementary particle in the Standard Model, plays a central role in precision tests of the electroweak sector and in searches for physics beyond the Standard Model. Its mass is a key input to global electroweak fits and is closely linked to the stability of the electroweak vacuum. In this contribution, the most precise ATLAS determination of the top-quark mass performed with the full Run~2 dataset at $\sqrt{s}=13~\mathrm{TeV}$ is presented, as well as several new measurements of top-quark properties. In particular, the first observation of quantum entanglement in $t\bar{t}$ production and two complementary tests of lepton flavour universality in $W$-boson decays are discussed. Together, these results highlight the increasing precision and broad physics reach of the top-quark programme at the LHC.