Learning from whoever is in the room
A young fighter's technical development is heavily shaped by the sparring partners available at their local gym, since those are the styles and problems they repeatedly practise solving. A gym stocked with pressure fighters will produce fighters comfortable working on the inside; a gym known for slick counterpunchers tends to produce fighters who default to range and timing, quite apart from any individual fighter's natural inclination.
House styles as an inheritance
Trainers who have built a gym's reputation over decades often teach a recognisable house style, a shared set of habits around guard, footwork and shot selection that gets passed to nearly everyone who trains there. Fans who follow the sport closely can sometimes identify a fighter's gym of origin from technical habits alone, long before knowing anything about their record.
Access compounds advantage
A gym's reputation also affects who is willing to spar there, since established fighters preparing for their own bouts prefer gyms with a strong stable of credible partners. This creates a feedback loop where well-regarded gyms attract better sparring, which in turn produces better-prepared fighters, making gym choice one of the more consequential and least visible decisions in a young boxer's development.