Bellingham official transfer to Real Madrid before the end of the month represents one of Dortmund most lucrative deals in recent years making it a stunning accomplishment. They can still achieve their main objective for the season thanks to this tactical choice even if they don't win the Bundesliga.
They have a solid strategy for the club from finding youth players, grow them up, build up their transfer values, then sell those players on transfer market, and repeat the cycle. They have executed that strategies many years and we can count many star players who grew up in Dortmund then leaving to big clubs: Sahin, Hummels, Lewandowski, Gotze, Aubameyang, Sancho, Haaland and maybe more. I can not remember all of those players they sold.
Maybe the word farm is indeed more suitable for Dortmund than Arsenal because we know that so far it has been Dortmund who have succeeded in growing some of these talented players. If Arsenal they are more focused on finding young players to build a team from scratch, so that as we see now they can compete at the top.
Arsenal relied on their young players and sold their star players in half of a decade because of their new stadium construction. They did not have enough budget for salary and transfer and they failed to convince star players like Fabregas, Van Persie, Song ... stay with them. After they completed paying debt for that new stadium cost, they changed and began to buy star players, competed for top four and titles again.
Dortmund is indeed a masterpiece as a reliable businessman and it's no wonder they will continue to look for young seed players until they finally get a lot of benefits from various top European teams. Haaland has indeed proven successful with Manchester City and it is not surprising that now his market value continues to skyrocket.
Dortmund is different than Arsenal as they don't want to spend big budget for star players. With very good scout team, they found many youth talented players around the world and grew them up. They continued to execute this strategy with belief that it is helpful for the club finance. The consequence is they are not competitive enough to beat Bayern Munich in Bundesliga.