Of course they're taxed. They have the same obligation to pay taxes as any native born citizen.
There may be more tax avoidance because they often work under the table but they're subject to the same penalties as anyone else if they get caught.
The only way they are taxed is if they steal someone’s social security number (a.k.a. taxpayer ID). And then they do pay payroll taxes. Otherwise they have no legal right to even hold a job in the US and thus cannot earn or pay taxes. Sales taxes don’t count because they are local and state.