Does anyone have a good website that explains how the elected US President forms a government? Where does he get the people to serve with him, in the various executive offices, when they aren't elected? How are they deemed 'fit for purpose' and who do they represent when they are serving?

Thanks. Do they all have to swear an oath to 'serve & protect the consitution, etc.'? How are the President's appointees accountable to the people?