How could they possibly do this? According to you, voters merely make suggestions and unless the law changes, nothing changes.

Because voters don't merely make suggestions. Who the President is matters, even though the President is not the sole person that the Executive agencies are accountable to. The President gets to direct a lot of policy - he's just not unconstrained, and staff is also accountable both to Congress and to private parties enforcing the law. That's why I keep pressing you to give me an example of an instance in the various agencies like EPA or Labor or Interior or Energy where the career staff just decided to "subvert" the President, rather than (say) conform to legitimate requirements of the APA or the oversight of Congressional committees or what have you.

But to clarify - when I say that if Trump sticks to it "they'll get the money," I meant from Congress. The GOP will do their Budget/Reconciliation package in early to mid summer. If Trump insists, you'll have funding in there to fund the staffing and construction costs for building new detention centers. A real "mass deportation" would probably require several tens of billions over the budget period - but if that's his main ask on the spending side, I think it's something he'll get.