You can view JHU's raw data sources at https://github.com/CSSEGISandData/COVID-19. According to that, they are getting COVID death numbers for Sweden from the Swedish Public Health Agency via https://experience.arcgis.com/experience/09f821667ce64bf7be6.... I am not fluent in Swedish, but perhaps someone who is can tell you exactly how the Swedish Public Health Agency measures them. Likewise for the other countries cited.