You know you've got an Academy when you can sell for £20m players who've never played for your first team. Chelsea (spot) excellent at this.
I think that comes down to two things: Chelsea being a big dog in the unseemly local youth market, and branding.
Chelsea can (and do) cast the widest net of all the local clubs. They have the longest-established networks and have spread themselves pretty wide from two bases, one at Stamford Bridge and the other at Cobham. Any kid near me who is good has at least flashed up on the Chelsea radar, and had a short trail at one of their many satellite youth programmes,. From there the best ones get filtered upwards until they are considered for the academy proper.
On top of all that, the market in the best academy youth talent is ferociously competitive, and Chelsea are aggressive. They have the money and glamour to entice players form other clubs and, for a club that size, can actually point to a good record of giving players a pro career, even if not at Chelsea.
The second thing is tied to the first. A player at Chelsea U19s is probably considered one of the best in the age group because they have such a strong youth set up. For that reason, if a Championship club wants to borrow a young pro they will turn to Chelsea first. Hence Gallagcher, Mount, Tomori, Geuhi, Abraham etc and so on, all getting a good start and becoming a valuable, saleable asset before or without playing much for Chelsea. In fact if it wasn't for a combination of a transfer ban and Frank Lampard wanting to give youth a go, most of them would never have played for Chelsea at all but would still have been a valuable asset.
Plus, there is the presumption that a kid at a big club is the best kid around in his position. Liverpool and Arsenal have a similar thing going for them: A young player who is on the fringes for them and gets the odd cup run out is seen as high-calibre by association, whereas one who can't quite get in our team is seen as not good enough.
Look at Gannon-Doak, Reiss Nelson, Smith-Rowe etc. Latest in a long line of largely average players who cost you at least £20m because of you are buying them from, rather than how good they actually are.
Chelsea et al don't actually have to put their youth team graduates in the team in order to get money for them. That's the key part of the business model. Clubs like us do have to. Its a massive difference.