Both. Sometimes younger players are touted out to clubs by their parent club, in which case most of the time the player will be known to a lot of youth set ups and clubs can decide whether it's worth making an offer.
Other times clubs scout the player and decide if they want to make an offer. For example with Hayden we've probably scouted him at an U21's game, but there'll no doubt be some dialogue between Bruce and Wenger.