Things are a lot more complicated than that. We cannot deport 20 million people. That would ruin the economy of a good many countries. We need to come to some sort of reasonable accommodation. Not all illegals want US citizenship, but a good many do.
We need to seal the border; build a wall. It worked in Israel; it will work here.
We need to know who is living in our country. That means a national ID card. Everyone should have finger prints and iris identification on file.
We need to sort the people out. Those here illegally that have criminal records get to go home and never come back. Those that don't have criminal records get to stay here and work and pay taxes.
Those here illegally get to choose. Either get legal or get gone. I suspect a good many will opt to return home. The prospect of paying US income taxes will send them packing.
Those that apply for US citizenship will get it eventually, if they can pass the test. Then they can vote. Those without citizenship should not get social welfare benefits.
Once these 20 million people are sorted out, those remaining will not be happy working for minimum wage. They will demand better wages and conditions. Things will change.
For what it is worth, any government official in Mexico can question one's immigration status. Perhaps that should be the case here.