As you raise the question of how reality can be one yet many, the man patiently listens. Once you are done, he nods sympathetically and smiles, before launching into his response.
It is concerning, isn't it? We regard "Being" as a unity because it subsumes anything we might mention. Our awareness requires some meaning or significance, therefore we cast the net accordingly and term everything within "existence". For if there was a "thing" that had an absence of meaning, we wouldn't be aware of it and we could not suggest that it was outside the net - for what would our awareness grasp? It wouldn't be a "thing" at all, we'd just be talking gibberish. So ultimately everything "is" in the exact same sense.
Your issue, as I understand it, is that if we say that everything is "Being", then how can the myriad things be different? For a horse is not a car, but yet both the horse and the car are Being. How can the horse not be the car, and the car not be the horse, when ultimately they're the same thing
The quickest way to resolve the issue is just to ask whether you admitted both "Being" and "Distinction" when you made that objection. For you are identifying both the horse and the car, so they are necessarily held together in common. Yet you claim that you are identifying two things, so you also necessarily admit of distinction or difference. So you have this awareness that both are true, which resolves the issue in a brutish manner.
That might not be intellectually satisfying, though. So let us discuss what else might be said about the issue. The key, I believe, lies in your use of the term "is not". Horse is not a car; car is not a horse. What do you mean by "is not" here? It must have some significance in the context of your question. Given that there is no alternative to Being, no "existential negation" so to speak, there appears to be only one option:
"Is not" shall be interpreted as "is other than".
When you are careful and ensure that "is not" is given clear meaning, the issue fades away. The horse IS other than the car, the car IS other than the horse. Both sides are subsumed by isness, which is the Whole, perfect and complete. That topic is my brother's specialty, of course; maybe I should tell him to include something about "is not" in his monologue, if he hasn't already done so. At any rate, for now you can see how the myriad of things are one, held in common by an absolute, omnipresent "is" that overflows all perceivable boundaries. It all just describes aspects of what-is.
Also, I think we should remember that we occupy a certain location or position within that context of what-is. Our conversation doesn't take place outside that context; everything we do is constrained by metaphysical necessity. How could you stare at one of the metaphysical axioms, entirely pure and alone? You can't, that would be impossible for a human. We can be aware of such things, but we cannot tease them out and give a pristine and contextless account.
I think we will soon revisit terms like "contradiction". As we continue the tour, you will hear of Order. There we have the context that gives "contradiction" its meaning; for without some logical system or shape to reality, what could contradiction signify? Being and Distinction are broader than Order, and therefore their relationship is beyond-contradiction; it truly can be what it is, free from that constraint.
Of course, you can just spend your days focusing on Being in its most universal sense, and avoid talking about Distinction. That's what my brother does, he stands around in that first room trying to perfect his monologue, using many words to consider the one. This seems inappropriate for a human, for we are products of Distinction and at some point we should examine the details.
If it's all a bit too much right now, you could always grab a bite to eat at the canteen. You might want to explore the grove, too, where a lot of our guests and adherents like to gather. I'll let you decide how to proceed.