the person applying for the green card already works for the interviewing company. the job opening is THAT green card applicant's job requirements.
the lawyer/law firm handling the green card application process has to prove that there are no US citizens who are qualified to do the job.
if there were qualified applicants for the job, then the green card applicant won't be given a green card, I assume. But that green card applicant is already working in the country via some other visa, so there is no job opening to fill typically.
the current person in the job is performing well, otherwise, why would you be trying to get a green card for them?
i've never heard of any green card applicant getting denied a green card due to a qualified US citizen applicant.