Primal summoning works exactly how the arcanist/summoner summons work.


Summoning requires 2 things, Aether and a piece of info.
In the case of the carbuncles the info is the gems , in the case of an egi/demi the info is the essence of a primal.
In shadowbringers we learn that to summon something you just need aether. As long as you have aether you can summon anything, this is how the tribes summoned the primals they simply prayed and used aether in the form of crystals. The reason it is harmfull and why we are unable to do it by just thinking is cause there is not enough aether around, The aether has been split into 13 different worlds/reflections . As for the echo in 5.2 we learned that the echo isnt hydaleyns gift it is our own innate ability from when we were whole , it is activated by simply witnessing a catastrophe , the reason it got attributed to hydaleyn is cause with the echo the first thing you hear is hydaleyns call , which is something hydaleyn does all the time its just that only people with the echo can hear it.

Id like to also say that every single ascian is an arcanist. Arcanima is efforts to express the nature of aether in mathematical terms that gave rise to the exacting science that we practice today. These definitive formulas are represented by "arcane geometries".
That means an arcanist can do literally everything, arcanist is to the world the equivalent of what a programmer is to a computer.