To whom is title "Professor Emeritus" given in the US?
In the U.S., "professor emeritus" simply means "retired professor".  It's a courtesy title offered to retiring professors to acknowledge their continued scholarly role even after formal retirement.  It typically includes library privileges and a computer account, and emeritus professors may also have offices (this depends on departmental policy and the availability of space).  It's sometimes possible to teach, but this is a special arrangement rather than a job requirement.

At many universities every retired faculty member is offered this title, with rare exceptions for people who committed misconduct or angered the administration.  At other universities one has to apply for the title and make a case for why it is justified.

In particular, professor emeritus is not a higher title or special distinction.  It can be viewed as a retirement incentive: even after retirement, you'll still have a respected role as well as necessities like library access.  However, there's no advantage to becoming an emeritus professor beyond the benefits of retiring from your job (i.e., giving up duties and focusing only on what you prefer).