> How anyone can become a billionaire as an employee is utterly beyond me.

You are thinking of all employees as fungible replaceable units, but employees are capitalistic units that can get monopoly power or economic rents.

An employee can provide a function that can’t be replaced, and that employee can capture a lot of the business profits, well beyond their actual “value” to the business. For example, Tim Cook’s personal relationship with Chinese officials might be very hard to replace.

Alternatively there is a kind of blackmail value, where losing a prominent employee can have high costs to the business, and the employee can get paid more than they are “worth” due to that. For example, perhaps Tim Cook leaving would cause more than a few billion damage in profits or valuation due to perceptions.