Well, I know you find this difficult to understand so I’ll try and help.
It’s a law enforcement operation which includes intelligence, they know who the coyotes and cartels are and the routes that they follow.
They also count the number of those they catch.
They base their estimates on those numbers. Typically the BP says they catch about 10% of those attempting to enter illegally.
You do not have to provide ID on the US side when exiting the country into Mexico so they have no way to monitor how many illegals are leaving voluntarily,