When MTB frames are designed, what takes priority?

Are your regular, mass market, trail/xc MTB frames are designed to ride well, or fit well, or a compromise of the two? Maybe just something your average joe can jump on and not feel uncomfortable, but handling characteristics are sacrificed.

Isn’t it pointless specing eff tt on DH/freeride/jump bikes?

When deciding on geometry, is it generally a consideration whether it’s a majority of in the saddle or out of the saddle?

Using stem lengths to adjust ride position, seems utterly pointless to me. Pays no attention to handling. But then maybe it’s OK if ride position is your priority.

Basically, I want a good old natter with some frame geometry geeks. I have a lot of questions.