This is how it works in France. You have to let the local authority know you intend to vote and they'll put you on the "electoral list" and tell you where it is you have to go vote. It's usually a school or similar not far from your residence. You can't just randomly show up to a voting place.

You can either go to your local town hall or do it over the internet (tick a few boxes, attach a proof of citizenship and residency in the town / district (for Paris) and they'll send you a confirmation).

Every citizen is automatically allowed to vote. Registering basically tells your local county that ‘I live and vote here’ so they’ll send you ballot information and have your name on the correct checklist at your polling place.

Canada is kind of in a middle ground. You need to register to vote, but when you file your taxes there's a checkbox for "Allow Revenue Canada to share your information with Elections Canada" which basically auto-registers you when you file.

>You register to be able to vote? Why isn't every citizen automatically allowed to vote?

Voting isn't a Constitutionally guaranteed right in the US, it's a privilege. Although the Supreme Court and various Constitutional Amendments greatly restrict the power of states to disenfranchise their citizens, states do have that power (for instance convicted felons often lose the right to vote.)

Also, voter registration laws have traditionally been an effective way of suppressing African American and immigrant voters[0] (who tend to vote Democrat) so red states tend to vigorously support such laws.

[0]https://time.com/5855885/voter-registration-history-race/