Under a presidential appointment, the Executive office can appoint someone to a position without a hiring process. In a civil service position, the individual generally (not always) will apply for the open position and then follow competitive hiring procedures against other qualified applicants.
However, it is disingenuous to state that presidential appointments are not based on merit as the President can not succeed if he is appointing individuals who are not qualified. Furthermore, in the federal civil service sector, there is an uncountable number of civil sanctions that have been levied against federal directors due to hiring policies that skirted merit and additionally violated federal EEO laws. (That is to say – nepotism and racism and sexism have routinely played roles in the hiring of civil servants)
To say, however, that all civil service positions are ‘merit based’ or competitive is simply not accurate and ignores a series of federal statutes known as the Schedule A, B, and C hiring authorities. These are NON-COMPETITIVE hiring authorities. That means, a person who has hiring authority can appoint a person to a federal civil service position without regards to merit or competitive hiring.
Of course, the President can hire and fire at will so when an individual is appointed by the President, that person can be removed from his job. When a person is appointed to a civil servant position under both competitive and non-competitive means, the person can not be removed from his or her position except in matters of gross negligence, criminal misconduct or medically incompetent circumstances.
Sadly, the process to remove a poor worker or a criminal is a very long and laborious process essentially making the removal of a bad employee incredibly difficult. The process for removing an employee who becomes disabled is a very simple and quick process and a blatant example of anti-disability bias. But, in general, civil service positions are lifetime jobs.