I think Nahiri and Ugin used the meditation plane to create the hedrons

As Jace states the meditation realm has immense creation potential and he remarks that ugin understood this while nicol bolas did not. Even if nihiri is the most competent lithomancer in the multiverse i doubt she could cover the entire plane of zendikar in hedrons on her lonesome. If jace could use the MR to make an entire multiverse, i think it makes sense that they could also use it to create the hedron network. If im missing some critical detail lmk.