At what systemic level do we start to see living beings making decisions rather than purely chemical reactions?

Frankly, the decisions that we make are chemical reactions. The difference is in the complexity of the decisions that we can make. At that point, though, in order to answer your question, we would need to argue about what one would consider to be a decision that's complex enough and a decision that's not complex enough, and that leans much more into philosophy and ethics rather than science.

I can only tell you that, from a mechanistic point of view, there's not really much distinguishing our decision making process from, say, the decision making of a flatworm

If I remember my cognitive science classes, all decisions even complex are powered by a person’s neural circuits, in which neurons talk to each other chemically using neurotransmitters. Basically, large groups of constantly-rearranging neurons are what turn what you call “chemical laws” into constantly-evolving decisions.

You might be interested in Tomasello's "The Evolution of Agency" where he kind of addresses this very question. It really depends on how you define "making decisions" and "purely chemical reactions" doesn't it - all life is chemical reactions, including when we make decisions, and it's easy for us to apply decision-making language even to systems that are simple enough that we can see them as "purely chemical reactions".

Tomasello defines the notion of "agents" as "feedback-control systems" that he distinguishes from pure stimulus-response systems. In his examples a nematode for example is "stimulus-response"; its behavior is very directly related to its immediate environment. If it runs into food it eats, otherwise it doesn't, and there isn't really a notion of it seeking out food when it's hungry and not when it's not. In contrast and "agent" is a feedback-control system with goals, a perceptual system that checks whether the goal is accomplished at any given time and a behavioral repertoire aimed at accomplishing the goal. In our lineage he sets the appearance of this agency around the evolution of vertebrates, and uses lizards as an example of the most basic level. (he doesn't address other lineages other than to say that various levels of agency clearly evolved convergently a few times; so octopuses and social insects for example would also have these systems). So where a nematode has feeding behavior that's triggered by running into food and other behaviors when food isn't present, a lizard's behavior depends not only on the immediate stimulus but on more abstract goals - in a given environment it might be currently hungry and looking for food, or sated and looking for shade or sun to rest or hide or thermoregulate, or looking to reproduce, etc, and its behavior will depend on and be directed towards accomplishing that goal.

It's interesting that you say "thinks through and makes decisions" as if they're on the same level but the book actually claims that human agency is actually the result of the evolution of several successive layers of feedback-control mechanisms that each allow more flexibility and responsiveness - so for example lizards have a feedback systems that adjusts behavior to achieve goals, and mammals have that and also a higher-level feedback system above that to adjust the goal-seeking behavior itself, mentally "playing out" different ways of accomplishing the goal in order to pick the best one. He describes four such levels for humans and it suggests a variety of ways we could define "think through and make decisions", with different species qualifying or not depending on which we choose.

You've stumbled upon the basis of the debate between free will and determinism. imo, we are merely under the illusion that we're making our own choices. The universe is one infinitely complex system of falling dominoes, with each choice and action just being the result of the parameters set by the ones preceding it. We are all made up of the same basic building blocks, and are thus just subatomic systems obeying the laws of thermodynamics... it just happens to be the case that when a system reaches a certain level of complexity, it is able to think about itself - we are quite literally the universe experiencing its own existence.

Why is this? I don't know. Nobody knows. Consciousness and 'the ability to experience' is one of the most elusive and complex questions facing science and philosophy today. It's my personal belief that there is certainly 'something' more to this whole cosmic experience, but I'm not convinced by religion's answers and believe 'it' to be something so vastly incomprehensible and foreign, we'd never understand it even if the mystery were revealed to us. It isn't something I like to think about too deeply, because unfortunately, it opens up an infinite regress of questions we will likely never have the answers to.

At some level, isn’t training just a chemical reaction? Perhaps never.

It's all chemical, and even bacteria, which are single celled, have quorum sensing.

This article about free will gives a nice introduction about your question!

Abandon the philosopical concept of an independent thinking mind capable of evaluating something by itself.

"Decisions" and "Thoughts" don't exist without the environment, as they're a succession of neuronal activation cascades in response to the current state of all stimulus, the previous connections formed in your brain, and reinforced patterns.

Leave a human being in an empty void and their thoughts will be built by severely boosting sensitivity and then responding to random sensory noise. Sever all sensory connections and the mind shuts down.

Quite frankly, all of them, as in literally all of the levels. e.g., viruses are not considered "alive" in the classic sense, but they sense things sometimes & change their behavior accordingly. A single protein can do it too, like in mad cow disease / scrappy (called "prions"). Even a tiny snippet of DNA can make logical circuits akin to computer ones, implementing AND, OR, XOR, NOR, operations etc., plus feed-forward loops (& feed-backwards, and all other sorts).

Possibly even subatomic particles, and maybe even quarks (or strings?) do the same - e.g. the Heisenberg Uncertainty Principle where you start to interact with something and then that changes it already so that you cannot measure other aspects of its previous "natural state". Okay that's not so much a "decision" as a "reaction" - but as you are questioning, what really distinguishes the two, REALLY?

Bacteria can sense a molecule (like sugar), literally start growing a tail (no joke!), and then swim towards it. All entirely chemically, and we have the technology to literally just kinda 3-D print all that at the molecular level (it takes an existing flagellum but once that is added to the mix, it can grow just like a crystal, by extension / copying of the old pattern).

Most of what we considered to have made humans "special" in the word turned out to be false - e.g. chimps & gorillas can "talk" (it's hard for their throats to make our kinds of sounds, but given the right apparatus they can get the job done), and think in abstract terms, and do math, and all kinds of things. Of course, humans ARE special - we are the only things on planet earth that if aliens came, could attempt to nuke them in orbit, and we literally light up the night sky! But there's a whole continuum of "dust" that share a lot of properties with us, in various ways. I'm not sure if animals have the same kind of subconscious vs. conscious interplay going on as we do, but if you have a pet and stare at it trying to work through a decision, you KNOW that it's doing the same as us, at a fundamental level. And then each time you go a level deeper, the similarities kinda never end...

Such questions may never even find answers, at least in our lifetimes, but it sure does seem worthwhile to ask anyway... it sharpens us, so keep digging!:-)