why some sportsbooks have way better player prop selections

Noticed lately that some books have a ton of player props, way more than others. I’m talking about stuff you barely see elsewhere, like defensive stats or weird combos that only hardcore bettors look for. Are they just getting their odds from different places or are some books building their own lines for every little thing? I get that bigger sites might have more resources, but it feels random which ones are loaded and which are just the basics. I try not to chase every little bet, but having more options is always nice. Anyone know what really drives the variety here?