Big clubs like Chelsea and City bring through players in their academies. The cream of the crop are then loaned out to other clubs, a few of the players then make a name for themselves at lesser clubs (normally in the Championship but some in lower level Premier Clubs) and are then sold for...
Academies now make money for the big clubs rather than supply players? Funding, to a degree, player purchases from other clubs. Occasionaly a gem may make the first team from the academy.