Here are some ways to talk about mind & body, each with some truth to them.
In immediate experience, the body happens within mind. The body is an experience happening within and as awareness. It’s a set of sensory input and associated imaginations (mental images and words) combining to create the experience of this body, and all of that happens within and as awareness. As somebody said, the body is the part of the mind visible to the senses.
Health wise, body and mind are one seamless system. If we decide to imagine them as two, we can say that each influences the other. (And the brain and body is one seamless system as well, obviously, with extensive two-way communication.)
In terms of ecology and cosmology, the body is a seamless part of larger social, ecological, and cosmological systems. It’s a holon (part) within a holarchy (system of parts). Again, any separation and even any distinctions are imagined. (And that imagination is essential for helping us orient, navigate and function in the world.) Our wider social, ecological, and cosmological systems are – in a literal and concrete sense – our larger body. This is not just poetry or the visions of a mystic, it’s literally how it seems to be. The universe is one seamless system, and we are seamlessly embedded in it.
In our culture, we are so used to thinking of separate units that it’s difficult to take this in and really embody and feel it. We are used to thinking of mind and body as distinct and largely separate, and ourselves as a person as distinct and largely separate from the wider whole. There is some truth to it, and yet it’s not the whole picture, and it’s also not the most useful way of looking at it these days. A more integral view, or systems view, or holarchical view, makes more practical sense.