I think there may be some confusion...
as far as I know, MOST of ESO is actually relatively small (small for Elder scrolls) self-contained sections of the continent. In other words, if you play the Daggerfall alliance, you'll spend ONE portion of your game in a basic starter zone, then you'll spend the next section in Orc Territory, then one section in Breton territory, etc. It is NOT one large, seamless open world.
during your first playthrough with a character from any alliance, you do NOT ever enter the territories of the storyline of the other two alliances - except, I suppose, in Cyrodill, where all the alliances eventually intersect.
apparently, once you've finished the story? or just reached a particular level (?) with your character, it will open up one of the other alliance storylines for you to play in (at least, that's how I understand it based on what people have said here), and you can play in another alliance territory with your character during THAT playthrough.
that's what I'm getting from everything I've heard here and elsewhere so far - if I've misunderstood it, or if I've confused you, I'm sorry, and certainly wouldn't mind a more detailed explanation myself.