The invariant law of nature which forms the basis of Buddha's Dharma and existence of universe itself

I am [rereading this answer](https://buddhism.stackexchange.com/a/47512/23920) and I am confused by the quote: >when this exists - that exists. Arising of this leads to arising of that. When this doesn't exist - that doesn't exist. Cessation of this leads to cessation of that. Let's suppose this is true, and let's consider an event X. By the statement, we should be able to find an event Y which caused X. Similarly for Y till ad infinitum. I think it is reasonable to assume that if we have a set of events in the present X_1,X_2,X_3..., when we repeat this process , all of their "root events" will start to become the same because things happen in context of each other. For example, I give my friend 5 dollars, then I also lose 5 dollars, now going back a step would go into the event of how the thought of me giving my friend 5 dollars came into our mind or my mind. Now, we apply extrapolate this idea through all events in the present, then it would be that they're all rooted at a very small set of events. And if one is to yet to apply the claim again, it maybe that all these events are caused by the same event. So, what would be these fundamental events which led the universe into motion according to buddhism?