Leptons are powerful tools to probe physics beyond the Standard Model (SM). The Belle II experiment installed at the SuperKEKB asymmetric energy electron-positron collider has collected a 428 fb$^{-1}$ sample of collisions during Run 1 (2019-2022). This data set, thanks to the excellent lepton identification capability and initial kinematics constraints, can be used to study lepton flavour universality (LFU) in semileptonic decays of the $B$ meson and $\tau$ decays. We present measurements of branching-fraction ratios that test the universality of $\tau$ and light leptons, using both exclusive and
inclusive B semileptonic decays and $\tau$ decays into lighter leptons. We also report the world's best limit for the search for lepton flavour violating decays such as $\tau^-\to\mu^-\mu^+\mu^-$.