as long as there is a single unemployed citizen, then we don't need the immigrant.

HOWEVER, we can modify that need by industry.
If we run short on doctors, b/c the citizens start trying to be rappers, for instance, then we allow a certain number of doctors to become citizens.

But when we talk about low skill labor, it isn't needed (in America); 21% of the country is on welfare benefits. Obviously, not everyone on welfare is capable of employment, and the spin on these numbers from both left and right is amazing.

In any case, every low-skill immigrant (whether legal or illegal) has the following effect (these are American figures, btw, other countries have different numbers):

• puts one low skill citizen out of work, and possibly onto welfare
• takes in more in government service than can be accounted for even if they pay taxes

the average U.S. household received $31,584 in government benefits and services

• remits a certain amount of money out of the economy back to their home country
• possibly goes on welfare themselves

It's not that immigrants are bad inherently; there has been a need for low-skill labor before, there may come a time it's needed again.

But immigration to the United States isn't going to solve Third World problems of poverty, especially those caused by native cultures (statism, socialism, kakistocracy, tribalism, and most importantly overpopulation). I don't mean "native" as in "primitive", but native as inherent to the culture.

Immigration certainly isn't going to fix those problems caused by globalization.

Finally, the best advice there is to fight poverty anywhere in the world, including America, is this:
don't have children you cant afford to feed

https://census.gov/newsroom/press-releases/2015/cb15-97.html