This is an excellent question because it highlights some of the difficulties of using language to describe external reality! It has one foot in philosophy because it asks, what is 'survival', and one in science because it is answered thoroughly by mathematical biology with survival analysis.
"Is survival a natural process that happens with the help of some x, y or z (I mean, help from something within and/or without)?
Short answer Is survival because of factors intrinsic or extrinsic to the organism? The answer is both, because by definition, biological survival means the perpetuation of the biological structure and activity (anatomy and physiology) by interacting with the environment by metabolic and reproductive activities.
Of course, the philosophical presumptions built into such a claim are contingent upon concepts that come to us from the philosophy of biology:
What is survival? In this case, from a physicalist perspective, survival is the perpetuation of biological structure from one organism to the next. That is, my biological structure through my reproductive system and that of a female's results in another human being. From a physical lens, this can be analogized with one domino that falls and hits another domino which falls and so forth. What we call an organism is a discrete unit of matter and energy that exhibits biological traits and cycles. Human development, though unique to every individual is generalized to the species. The main difference between human reproduction and falling dominoes occurs at the thermodynamic level, because life is 'negentropic' to use a term by Erwin Schrödinger. That is, dominoes move from order to disorder, whereas human life moves from less order to more order. This process is not fundamentally different than how water from from liquid to ice and back from ice to liquid other than in terms of complexity. Obviously the biochemistry involved dwarfs inorganic chemistry of water's phase changes.
Inside and Outside It is a an accepted precept among cognitive linguists that the notion of Containment (inside vs. outside) is a conceptual metaphor. A conceptual metaphor might best be understood as an ontological primitive. Our brains behave in such a way that inside and outside and surface and volume give meaning to our subjective reality which is imposed on the causality between us and not-us. But mathematics and science have shown that these intuitive notions of definite inside and outsides fall apart on inspection. For a two dimensional example, take the coastline paradox. One can stuff infinite lengths inside of finite area and create other odd geometrical objects that violate our intuitive notions because they manifest the same properties of infintesimals. Remember, that any measurement of the physical universe imposes normativity on the universe, a fundamental principle easily revealed in the extreme by wave-particle duality.
So, the question of inside-or-outside leads to the question of where does the inside end and the outside begin, and that becomes a philsophical problem in and of itself. 'Inside' and 'outside' are always approximations with a precision. Intuitively, most would except that a grape on the hand is outside the body, but in the stomach is inside. But at what point is it in? More than halfway in the mouth? A topologist could claim it is never inside the body at all because topologically, from mouth to anus, we resemble the torus. A biologist might look at the same path an claim the intestine is outside the body because it is on the outside of the skin and the circulatory system. An anatomist could pull floss through the mouth, out the anus, and tie it together to show the how the orifices are connected.
Higher and Lower Creatures Third, let's draw a map of what this means. When you say brain, you are oversimplifying the biology at play. If we broaden 'brain' to 'nervous tissue' we can see the continuity from the simplest of organisms to the most complicated. The brain, then, is a special case of how nervous systems function to allow an organism to adapt to the environment. Simple cells may respond based on simple chemical gradients and have no nervous system at all, but as biological structure becomes more complicated, cells start to specialize in such a way that they exhibit biological teleology. In this manner of representation, we can describe the adaptability of life as a general feature, or in philosophical parlance, a necessary criterion of life. So certainly, in survival analysis, when one is examining various factors, survival is clearly a function of the sophistication of the complexity of the nervous system. Human adaptability is generally seen as the most sophisticated form because of its use of language and other eusocial features, a thesis explicated at length by the great thinker Edward O. Wilson in this work. But, put a man in space without a spacesuit, and no manner of cleverness will lead to survival.
Ecology and Holism Fundamental to the factor analysis of biological survival then is not just the factor of what is "within" the organism, but rather an interplay between the organism and the environment, which is the primary focus of the biological subfield of ecology. So, from the mathematics of survival analysis, we must not only give credit to a human's complex agency, but those factors in the environment that allow the human organism to survive. Is the air clean? Water present? Pathogens? Other organisms competing for resources? Cosmological events such as meteor impacts? All of these environmental factors contribute to survival, which ironically is precisely what gives our brain the advantage: we can identify positive and negative factors and will them into and out of being. Smallpox is a bad factor so we extinguished it (mostly). Grain is a good factor so we have leveled nature and replaced much of it with fields for us and our livestock.
