This question first depends on how you define “machine.” Since it is in quotation marks, it should be a metaphorical usage. I can hypothesis that you mean an entity that acts according to certain programs and instructions. If my hypothesis is correct, then in many cases a human being can be considered this kind of “machine.” But if what you want to explore is whether humans truly have free and independent will, then perhaps we can discuss that separately later.

At the physical level, yes. But there are some important qualifications. For example, every metabolic process in your body follows the laws of physics and chemistry. You cannot prove that there is a non-material “life force” inside the human body. At the level of atoms and molecules, you really are just a collection of physical processes. But some features emerge at higher levels of organization, such as consciousness, emotions, and thoughts. They are not separate from the physical—instead, they emerge from complex physical arrangements. The hardest problem is that many people admit we humans have not yet fully explained how subjective experience emerges from the physical.

I think it can. Everything inside our bodies, from cellular reactions to neural signal transmission, follows physical and chemical laws. Even our emotions and thoughts essentially come from chemical reactions in brain cells. In principle, all human functions can be broken down into physical and chemical processes, like an extremely complicated machine.

A human being can be “understood” as a complicated machine, but cannot be explained as a complicated machine.

If you look at it from a scientific angle, yes; from a philosophical angle, some people may have other views.

I think it cannot from the perspective of the universe, but it can be plausible from our human perspective. First, from the very beginning, science, physics, and chemistry are invented by humans, and we have our own way of understanding the world and ourselves. With the knowledge we have, at most we can understand how our bodies work as different subsystems, how they behave, interact, and exchange information. But this is still a human-level understanding.

In theory, everything, including human beings, can be explained by a physical model. However, the degree to which we can be predicted is worth debating. Chaos theory suggests that deterministic theories are practically unpredictable due to extreme sensitivity to initial conditions. A “machine” modelling a human would be unreasonably complex: every single cell and every single neuron firing must be considered. The sheer number of variables would require an enormous amount of computational power, which would be impractical in our universe. Therefore, just as we cannot predict the weather perfectly, we cannot fully predict a human being either.

In principle, a physicalist could say yes: every biological process may ultimately depend on physical and chemical processes. But a complete account of a person may still require higher-level explanations of consciousness, psychology, and social life; whether these are fully reducible remains disputed.