How is the national electoral register created?

Spain, Office for Democratic Institutions and Human Rights need assessment report, 'Early parliamentary elections, 28 April 2019', Voter Registration , accessed 2 July 2019

Voter registration is passive for in-country voters and active for voters abroad. Citizens above 18 years are included in the voter register are eligible to vote. […] The Electoral Census Office (ECO) is responsible to compile the voter register. Voter lists are comprised of voters residing in-country and abroad and updated monthly on the basis of information from the Civil Registration Office, and the Ministries of Justice and Foreign Affairs, European Union and Cooperation.