To learn more about the Decennial census, along with other data collecting activities of the US Census Bureau, visit Amherst College's LibGuide entitled "Find US Census Information" created by Dawn Cadogan.
The US Census is the basis by which seats in the House of Representatives are apportioned. The Apportionment Act of 1941 created the "the method of equal proportions" which distributes congressional seats by population (as established by the census) following the awarding of at least one seat per state. This process of reapportionment occurs after the decennial census, which happens every 10 years in years ending in "0". The census counts all residents of the United States, both citizens and non-citizens, but does not include residents outside of the states in apportionment calculations (such as those in the District of Columbia and Puerto Rico). Federal employees residing abroad are also counted. You can view the 2020 Census reapportionment data online. While the federally conducted census informs apportionment, there is no federal legislation outlining how House seats should be allocated within each state. Therefore, redistricting is governed largely by state laws and party politics with some federal oversight (due to historic repression of marginalized peoples' voting rights). COVID-19 delayed the census which has effected reapportionment and redistricting, as reported by Politico.
Federal law relating to apportionment:
"The census, apportionment, and redistricting are interrelated activities that affect representation in the U.S. House of Representatives. Congressional apportionment (or reapportionment) is the process of dividing seats for the House among the 50 states following the decennial census. Redistricting refers to the process that follows, in which states create new congressional districts or redraw existing district boundaries to adjust for population changes and/or changes in the number of House seats for the state. At times, Congress has passed or considered legislation addressing apportionment and redistricting processes under its broad authority to make law affecting House elections under Article I, Section 4, of the U.S. Constitution. These processes are all rooted in provisions in Article I, Section 2 (as amended by Section 2 of the Fourteenth Amendment)." Sarah J. Eckman, Apportionment and Redistricting Process for the U.S. House of Representatives, Congressional Research Service (2019).
"Congressional redistricting is the drawing of district boundaries from which the people choose their representatives to the U.S. House of Representatives. The legal framework for congressional redistricting resides at the intersection of the Constitution’s limits and powers, requirements prescribed under federal law, and the various processes imposed by the states. Prior to the 1960s, court challenges to redistricting plans were considered non-justiciable political questions that were most appropriately addressed by the political branches of government, not the judiciary. In 1962, in the landmark ruling of Baker v. Carr, the Supreme Court pivoted and held that a constitutional challenge to a redistricting plan was not a political question and was justiciable. Since then, a series of constitutional and legal challenges have significantly shaped how congressional districts are drawn." L. Paige Whitaker, Congressional Redistricting: Legal and Constitutional Issues, Congressional Research Service (2016).