HID of MAXI J1820+070 during its 2018 outburst (from MJD 58 189 to MJD 58 305). The hardness is computed as the ratio between the 4–12 keV and 2–4 keV NICER count rates. On the y axis we report the total 2–12 keV NICER count rate re-normalised for the number of used FPMs. Highlighted in different colours and symbols are the phases of the outburst that are the focus of our work. The grey empty circles show the subsequent spectral evolution of the source up until MJD 58 404. The epochs of the appearance of a type-B QPO (MJD ~ 58 305.7; Homan et al. 2020) and the launch of relativistic discrete radio ejecta (MJD 58 305.6 ± 0.04; Wood et al. 2021, and ~MJD 58 306, Bright et al. 2020; Espinasse et al. 2020) are marked, respectively, by the bigger star and orange arrows.