Tau and low multiplicity physics at Belle and Belle II

The Belle and Belle II experiments have collected a 1.4 ab⁻¹ sample of e⁺e⁻ collision data at centre-of-mass energies near the Υ (nS) resonances, of which 424 fb⁻¹ were collected at Belle II in Run1 (2019--2022). We present a measurement of the lepton-flavour universality between electrons and muons, the search for the lepton-flavour violation decay τ⁻ → μ⁻μ⁺μ⁻, and a measurement of the e⁺e⁻ → π⁺π⁻π⁰ cross section in the energy range from 0.62--3.5 GeV using an initial-state radiation technique, all performed with Belle II data.