> What does it mean that they underpay their workers, except that they pay them a smaller percentage of the economic value produced by their labor than you think they should?

That's easy to measure. These companies have a per-employee revenue which is easily a 5-10x multiple of average total compensation paid per employee, and in some cases it's also much less than the per-employee profit.

So they could pay a ton more while also making a healthy profit. Apple is the worst example of this.