If we look at iPhone, the flagship iPhones get specific features in the silicon tailored for iPhones specifically, like hardware accelerating computational photography and the new display engine in the A16 to make the Dynamic Island animations smoother.
On the Mac side, the highest end Macs get additional M-series features like high speed memory bandwidth And of course more cores, better graphics, RAM, etc.
How about the flagship iPads? Well, they only get whatever the MacBook Air gets. The iPad Pro is not in the driver seat in any way. It gets software features first developed for the iPhone and it gets whatever new silicon that is developed for cheapest Macs. The iPad has no agency, both on the software side and the hardware side. It’s truly the third wheel.