the Loom is the infinite canvas upon which All-Under-Creation is woven. think of the "stuff" between it as a mist-shrouded melange of open sky and open sea, shifting seamlessly between the two, crossed by all manner of boats. the Spheres are those worlds that people live in; some entire planets, some tiny islands.
picture not a flat earth but a flat universe, with worlds as continents and space the oceans between them. if that makes sense?