Focusing on the "illegals taking jobs" argument is almost like straw man hyperbole because it is only spotlighting one facet of the debate.
People choose to focus on that point because it is easily defended with perception rather than hard data and it lends an answer that supports their cause.
Would most Americans do the jobs that many illegals take when the arrive in the US?
-No
However, asking that question is ignoring the REAL issue that most US citizens wouldn't do that job that illegals do -
for the wages they do it.
Most Americans would do those jobs if they paid more, and they would have to pay more if there wasn't a readily available supply of cheap labor.
Illegal immigrants increase the supply of cheap labor and therefore put downward pressure on wages in the US. It's simple supply and demand economics 101. The greater the labor force relative to available to jobs, the lower the wages will be.
The abundance of cheap labor had been a large factor in minimum wage not keeping pace with inflation.
To put it in very simple terms. If I have a job opening and am offering $10 per hour and no one takes it what do I do? Increase the offer to $12 per hour.
Now if in the same situation I receive a dozen offers what do I do next time I offer the position? Decrease the offer to $8 per hour.
The supply of workers puts downward pressure on wages, which negatively impact ALL working people illegal or legal.
In addition, from a simply altruistic POV, illegal immigrants are more exploitable then legal citizens both because of a lack of support and information, so many of them end up making below minimum wage and in some cases end up in modern day indentured servitude because they don't have the protections that legal citizens enjoy.
Many big business DO NOT support immigration reform. Why do you think that is? Because illegal immigration is good for big business.