Adjustments to match the census population with the UN estimates are applied at the national level. The adjustments are made to match the UN country population estimates for the years of 2015 and 2020. The UN estimate for a given country (or state/territory) is divided by the total census estimate of population for the given country. The resulting adjustment factor is multiplied by each administrative unit census value for the target year. This preserves the relative population totals across administrative units while matching the UN total.
More information, including which countries use 2015 vs. 2020 estimates, can be found in this Excel file.