The answer is the federal registration form, (motor voter), which does not require proof of citizenship, just an “affirmation” under penalty of perjury that the person is a US citizen. The Dems have fought individual states that have asked to have proof of citizenship included on the fed forms for their states but have been denied by the agency in charge of these forms and Dems have fought tooth and nail in the courts to deny the changes to the form. Under the law, states must accept these registrations on face value and register these “voters”.
To expand on this. PDJT had a voter integrity task force right after the 2017 inauguration. They were asking the states to give them their voter rolls, (with the non-public info such as DL’s and SSNs), so that they could audit the illegal voters on the roles along with the dead and moved voters. Most of the Dem states refused to give it to the task force. This election chaos may, very well, allow PDJT to get a court to order those records to be given to him for his team to verify the fraud in the voting system of these states.