“how can private 501 C do that?”
Obviously, they can not exercise eminent domain.
As a 501 C 4, they could spend the money to build some walls on private property, which the government will not maintain.
They could also spend the money on lobbying, instead of building.
If, as a 501 C 4, they are found transferring substantial wealth to someone controlling it, they will have to pay a steep excise tax on those transfers. But they can do it, and it would clearly pay well.
As a 501 C 4, they could spend the money to build some walls on private property, which the government will not maintain.
It will drive the leftists crazy when California, Arizona, New Mexico, and Texas farmers and ranchers start building the wall on their own property to keep the invaders off their property.
For years we have heard the stories about these border state ranchers and farmers being afraid to travel their own property because of the foreign invaders.