Ads(literally on instagram), posts on the main gov hiring website, campus career days/events, and generally name recognition.
Depends how senior of staff but most promote internally. Director levels are appointed.

Spooky agencies who do Intel things do reach out to promising candidates, high language skills, knowledge of necessary areas, specialized expertise like certain ex-mil jobs. A common theme was to have agency contacts at schools and have them point out promising people to recruiters.