I looked and I can't actually find a good answer to that.
I do know that the job requirements are basically:
1. US citizen
2. High school diploma
3. 18+ years old
4. Pass drug/background screening
For the pay, the job requirements are very low. They also follow the "Veterans preference" practice during hiring. This is actually pretty typical across many government jobs (See https://www.usajobs.gov/).
My experience with trying to apply to these jobs (admittedly second hand, watching my wife apply to meet grant-related conditions) is that if you're an honorably discharged veteran, your resume goes into bucket #1, and if they can fill the position from that bucket, they won't even look at bucket #2, which is public applications.
