Most of that is difference in the process technology. Snapdragon 865 Plus uses N7P, A15 uses 5nm. It's not Qualcomm vs Apple, It's TSMC vs TSMC. Apple pays premium for first digs.

Rest is microarchitecture design choices and integration. Apple designs only Apple in mind and can afford to choose expensive solutions for performance gains. Qualcomm processors are sold for a larger number of customers and they have to compromise.