In my hellscape there are three sources of information about who lives in country

1. Registration in police

• They also issue internal pasports
• I think foreigners register there too
• Their registration is used by election committees
• Registration is based on place where you live(permanent for your home and temproary for stuff like hotel, camping site, prison…)

2. ЗАГС

• Civil registration?
• They give birth and death certificates
• They also keep track of citizenship
• And marriages

3. Census

• They knock on doors and write whatever what said by people
• They ask foreigners only 7 questions and 33 questions for citizens
• If you say you are unicorn mare from Equestria they will write that here lives unicorn mare from Equestria
• It is used to get factual information about what people live here
• They don’t ask pasports or any other document

Participating in census is not mandatory here. If you make census mandatory it will probably be no better than existing beaurocracy and defeats purpose of getting on-ground truth.