It is a longstanding Catholic belief that God works through secondary causes. At one time in salvation history it was thought that God worked to cause everything directly. (I think the Muslims may still hold this view). So if you got a cold God caused it. Or if the sun rose in the sky then God was directly causing that too. However, as our understanding of the world and of God matured theologians understood that God works through secondary causes, while still remaining the first cause of all that exists. This means that God doesn’t have to directly cause everything that happens in the world in order for it to happen. He can create certain natural laws that material things must obey and have certain patterns of behavior that they must follow. For instance the behavior of atoms and molecules. Or the law of gravity and entropy.
So if you are asking when secondary causes stop and God starts this is something that Aquinas talked about in his 5 ways. We can see God as the First Cause in a chain of secondary causes. This is illustrated by considering a coffee cup being held up by a table. The table is holding up the cup. So the table is the cause of the cup being help up. But, what holds up the table? The floor is the cause of the table being held up. Similarly, the earth is the cause of the floor being help up. In this example, we can consider the earth to be the first cause of this sequence of causes. And, there can not be an infinite number of secondary causes without a first cause. Since without the first cause none of the secondary causes could have any power to hold up the cup. Each secondary cause derives its powers from the the first cause, the earth. The table derives it power to hold up the cup from the floor which derives it power to hold up the table from the earth. Thus, without the earth none of these secondary causes could have any powers to hold up the cup.
Similarly, when I consider what is the cause of my existence at any moment, it is similar kind of process that must have a first cause. For instance, I could say the cause of my existence is the organs working together in my body. But, what is the cause of the organs? I could say that their cause is the cells in my body. I could continue this line of reasoning and say the cause of my cells are the atoms that make them up. And the cause of the atoms is the quarks and so on. Ultimately, like in the cup example, I have to come to a first cause. Now, the first cause in such an example must be something that is uncaused and for which its essence is its existence. That is its essence is to exist. Since it can not derive from anything else or it could not be the first cause of my existence. So this first cause must be an uncaused cause that is existence itself. It must be eternal and uncaused. This is what we call God. So God is the ultimate or first cause of my existence at any moment, from which all other secondary causes derive their power.