Ultra-low-background physics: lessons from BOREXINO and future steps

The Borexino experiment concluded the data acquisition at the end of 2021 and among the solar neutrino experiments it has been the only one capable of reconstructing the position and the energy on an event-by-event base, with an energy threshold of 150 keV, thanks to its ultra-high radio-purity. The experimental techniques and analysis methods that allowed Borexino to reach such unprecedented levels of radio-purity are now a standard and the legacy that Borexino leaves to the next low energy neutrinos and rare event searching experiments. This contribution is aimed to present the methods and the main achievements of the Borexino and to summarize the broad experimental effort presently in progress in the field of ultra-low background physics to further improve the sensitivity with massive underground detectors and new techniques.