1. If, say, an immigrant earns 70k, they will also spend 70k. It's an equilibrium - they "create" as much jobness as they "take".
2. The easiest intuitive explanation I have is - imagine a million immigrants move in. They're basically a self sufficient city - with police and doctors and teachers and mechanics and salespeople and whatever. Doesn't bother me.
In fact, America is by definition (unless one is native American) the land of immigrants. It is clearly factually historically obvious that immigration was not zero sum and it continually expanded overall prosperity and might of the American empire. It's only when we zoom into the very here and now that some people, for some reason, freak out and feel "omg immigrants!!".