Due to its large branching fraction, Higgs boson decays into pairs of W bosons are among the most promising signatures to measure CP and coupling properties of the Higgs boson, as well as its inclusive and differential cross-section. The leptonic final state $\ell\nu\ell\nu$ provides a clean signature and is efficiently selected with lepton triggers. The combination of a high rate and a clean signature provides an opportunity to measure all the major production modes (ggF, VBF, WH, ZH) in a single decay channel. The studies presented here are based on the proton--proton collision data recorded by the ATLAS detector at the LHC at a centre-of-mass energy of 13 TeV with an integrated luminosity up to 139 fb$^{-1}$ data. All the measurements are found to be in agreement with the Standard Model.