code: def populate_countries(self):
          
          for i in range(1, 12):
              self.admin_level_comboBox.addItem(self.tr('Level %s') % i, i)
  
          # Set current index to admin_level 8, the most common one
          self.admin_level_comboBox.setCurrentIndex(7)
  
          list_countries = sorted([
              self.tr(country) for country in list(self.countries.keys())])
          for country in list_countries:
              self.country_comboBox.addItem(country)
  
          self.bbox_countries = {}
          for country in list_countries:
              multipolygons = self.countries[country]['bbox']
              self.bbox_countries[country] = []
              for coords in multipolygons:
                  bbox = QgsRectangle(coords[0], coords[3], coords[2], coords[1])
                  self.bbox_countries[country].append(bbox)
  
          self.update_helper_political_level()
docs: Populate the combobox about countries and levels.