Example:
1. Where were you born?
2. Were you or are you to be naturalized?
3. Are you a legal immigrant?
Etc.
Ascertaining the citizenship status goes into the question of calculating the APPORTIONMENT POPULATION. That's the official term.
Only citizens are entitled to representation in Congress.
The Apportion Population determines the number of seats allocated to each state.
The Constitution does not mandate how the Apportionment Population is to be formulated for calculation.
In effect POTUS is saying that the Apportionment Population will be estimated from known federal records and we can be assured those records will not allow illegals to be included in that apportionment calculation.
If Pelosi and he Democrats try to stop Commerce from developing estimation algorithms based on federal records, she will need to pass a law which she will never be able to do.
However, this estimation curve ball must not set a precedent. After the 2020 elections, relying on the GOP to take back the House and hold the Senate, there must be a law authorizing the citizenship question just to clarify so that the shenanigans of Justice Roberts and the Democrats cannot again try to skew the Apportionment Population to illegal populations to further their hold on power.
See response 71 and 72