There is a certain amount of, well, call it clutter, in your question.
The usual attitude towards scientific laws is not that they are true. It is that they work for all known observations, we have good reason to think they could work over a much wider range of observations, and no reason to think they could not work over an arbitrarily wide range of observations.
OR a scientific law has been shown to have a constrained range of applicability within which it still operates correctly. We shall see some examples of that in a moment.
It has somewhat the flavor of logical positivism and the work of Popper. Though there are many statements of the plan. A scientific law is an idea that has not yet been disproved, and that has gone through a lot of determined and clever efforts to do so. Or it's range of applicability has been carefully mapped out, and within its range it works.
However, science is always prepared for our cherished ideas to be trashed by the next observation. And the reason we are prepared for that is because it has happened on so many occasions. We once thought the Earth was flat and the Sun went around the sky. Then we thought that the Earth was a ball and it went around the Sun, but we had little idea about gravity. Then we got Newtonian ideas about gravity, and we expanded our base of knowledge. Then we got observations that were trouble for Newton, and Einstein came and rescued us with relativity. Then we got into observations of atoms and nuclear reactions. And various people in the 20th century brought us quantum mechanics. Heisenberg, Schrodinger, Feynman, Weinberg, and quite a few others.
The point of this series is this. Our basic understanding of the nature of reality changed at each step. From a geocentric idea at the start. Then a heliocentric idea. Then an absolute space and time that we got shiny and new from Newton. Then a relative space-time that can change and dilate and warp. We got that still in its original packing straight from Einstein. Then a quantum foam that we got from the quantum mechanics folks, which we still have not completely managed to get out of its packaging. And these days, we are playing hard with stringy reality. Though so far that has not been as successful as one might hope.
Within the bounds of a couple of kilometers, you can probably work with the idea that the Earth is flat and use "scientific laws" of geometry that we got from Euclid. The curvature of the Earth will not affect your marking off a rose garden to any great degree. Within speed limits of very much slower than light, and where gravity is not important, you can probably stay within Newtonian physics and use Newton's laws of motion. If you are not too accurate you can probably accept the results of Newton's law of gravity. And if you are looking at only the orbital motion of objects in the solar system, you can accept Einstein's theory of gravity. (Be careful about galaxies and dark matter. But it's a fun activity and the people working on it have pizza.)
A personal annecdote, possibly apocryphal, but it gets the idea across. There was this physicist who had been working on his theory for 20 years. He had been publishing papers, training grad students, directing experiments, and he was quite well known for his body of work. One day, a visiting lecturer gave a talk on his results on the Great Man's theory. And he proceeded to show, quite clearly and definitively, that it was wrong. The audience was nervous, glancing at the GM. Who was scowling and frowning through the whole talk. At the end of the talk there was a pause and "Any questions?" And the GM stood up. And he walked up to the visiting speaker. And he shook his hand, grinning widely, and exclaimed: "Thank you sir! You have taught me something today!"
We hope we are getting closer to reality. We hope we are refining and not just thrashing. But we do not expect that we have found absolute final truth. We are always prepared for our current best ideas to be utterly replaced. Indeed, we are kind of hoping for it. Such events mark advance in human knowledge. We hope to be there when it happens.
There are ideas we are more confident of than others. Often we elevate such ideas to the status of scientific law. The attitude of modern physics can be expressed by a quote from Einstein.
“What really interests me is whether God had any choice in the creation of the World.”
― Albert Einstein
There are ideas that seem to be inescapable within the range of observations we currently have. Just as an example, it seems very difficult to avoid Unitarity, although there are speculative theories that explore the idea. This is a very technical thing from QM. But basically, it is difficult to conserve the quantities that we see conserved unless they evolve by a unitary operator.
There are those who think that, given some very basic observations about the universe, a theory based on unitarity is unavoidable. That is "God had no choice." It is a challenging notion.
This brings us to your example of conservation laws. Conservation laws arise from symmetry. We know this because of Noether's theorem. This is one of the Great Results of the 20th century.
The basic idea of Noether's theorem is this:
If a system has a continuous symmetry property, then there are corresponding quantities whose values are conserved in time.
The maths to understand this are graduate level. And 40 years in my past. However, there are several easy examples.
- Conservation of momentum arises from space-translation symmetry.
- Conservation of energy arises from time-translation symmetry.
- Conservation of angular momentum arises from rotational symmetry.
- Conservation of electric charge arises from gauge symmetry, a feature of the electro-magnetic theory of Maxwell.
- Some quantum numbers (such as the number of certain types of particle) are conserved, and this conservation arises through unitary symmetry.
Fundamentally, Noether's theorem is geometry. The space in which that geometry operates is usually the phase-space of some physical theory, but still, it is geometry. And it is a symmetry argument, one of the strongest arguments we have in all of science.
So, according to these notions, conservation laws arise as so. There is a thing we call reality. It has symmetries. For example, it is symmetric under changes in the origin of location. By that is meant, the behavior of the universe does not change if we set the origin of the x-axis here or instead there, although our angle of viewing may change. Then we do a huge amount of mathematics. Then we conclude that this symmetry implies conservation of momentum.
So, there is no abstract thing to interact with existents. There are objects in reality and their characteristics. If an object has rotational symmetry then it will have conservation of angular momentum. If reality does not have a privilidged time, meaning the origin of time can be set arbitrarily, then energy is conserved. And so on. It is properties of existents, not abstract things, that interact.