It is public. Universities almost always include the donation size when they put out press releases about the new building.

It isn't public - none of these universities publicly say, "You can donate 2 million and we'll let your kid in". Which was the amount at Stanford in 2012 (at least based on what someone I met working in admissions told me).

It's left unsaid because it's better for the brand to pretend that this isn't a thing and that just because your parents gave money or because your parents are famous doesn't mean they'll let you in.

But very rarely how many of the donor's family they lowered admission standards for, and by how much.

And usually the building bears the name of the donor!

Often they take the classy move and name it after a relative, often a parent. Harvard's Maxwell-Dworkin building got that name because those are the maiden names of Bill Gates's and Steve Ballmer's moms.