Undocumented just means an immigrant who is likely here illegally. There are ways to determine the amount of people currently living/residing in a city without them having officially registered with the government (e.g. census surveys historically never cared if a person was illegal or not, they only cared about counting people).

They are undocumented, that's why it's an estimate.

Honestly not sure if serious or a joke, but I've seen things today that make me believe anything.

There’re estimates, but since we want to play coy and side with fascist dictators who are using special military forces against their own people for not voting for them:

If the problem is people crossing the southern border, why is ICE in the most northern states?

Ok, now argue the actual point.

You just make a list of all the documented ones and then circle the ones that are not on the list.