Thank you for your question!
According to the resource I found below
https://study.com/academy/popular/how-to-pass-the-civil-service-exam.html
the United States no longer administers a single overarching civil service examination.
Some branches of public service (the Post Office for example) do still utilize a form of civil service examination which is more tailored to their specific job needs.
In conclusion and in answer to your question, if you were interested in a particular public service job, you should apply to that job locally, and could then be directed to a study guide and resources specific to that profession.
I hope this answer helps you!
Have a great day.