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, then shares his thoughts with you.
It's concerning, isn't it? We regard "Being" as a unity because it wholly 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 lacked any meaning, we wouldn't be aware of it and we could not suggest that it was outside the net. What would our awareness grasp? How could it fall outside the net, which encompasses everything we can reference? We'd be spouting gibberish.
Your issue, as I understand it, is that if we say that everything is "Being", then how can the myriad details be distinct? For a horse is not a car, but I am claiming that 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 things, so you accept they all exist in a broad sense. You also claim that you are identifying two things, so you necessarily admit of distinction or difference within that broad context. So you accept that both are true, which at least gives us the undeniable conclusion, albeit in a brutish manner.
That's not intellectually satisfying, though. So lets discuss what else might be said about the issue. The key, I believe, lies in our use of the term "is not". We must explore negation. Horse is not a car; car is not a horse. What do we mean by "is not"? It must have some significance in the sentence. 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 we are careful and ensure that "is not" is given 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, that principle of omnipresent existence. 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 considered it. 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 the Whole.
Also, I think we should remember that we occupy a certain location or position within the Whole. 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, 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" a clearer meaning; for without some logical system or shape to reality, what would contradiction signify? Being and Distinction are broader than Order, and therefore their relationship is beyond-contradiction; it is what it is, free from narrow constraints and expectations.
Of course, you can just spend your days trying to focus 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 one. This seems inappropriate for a human, for we are also products of Distinction. We cannot escape our position alongside the multitude of 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.