They call it THE Grid because it is definitely the earliest surviving, and possibly once the most powerful, magical network to ever be in existence on Earth. Some mages don’t bother with playing it safe by saying ‘possibly:’ while They have certainly produced networks of similar potency and subtlety, it’s arguable that THE Grid simply had more reach. Unfortunately, there’s no way to tell at this point, as THE Grid is at least 70 million years old. Which, yes, makes it of pre-human origins.
THE Grid is absolutely not active, in the sense that more modern magical networks are; whatever central power nexus or linked nexi existed back then have long since sublimed back into the aether. What’s left are largely nodes: permanent minor bits of space-time that have been folded to allow ambient magical energy to collect and accumulate. This is typically only enough to allow nodes to remain magically entangled with each other, which effectively allows mages to use them to communicate cheaply with other mages in the presence of a node.