The "fire" on the surface arises because the core is really, really hot. If something has a hot interior, then its exterior will also get hot, even if not as hot as that interior is. You can see this in an everyday situation by thinking about an oven. If the oven has been running for a while, you will probably not want to touch the outside surface of the door, even if that is not the inside surface and not as hot. Heat transfers from the hot center to the exterior through the intervening matter in a number of ways, and the same is the case with a star like the Sun.
And at 15 million kelvins, that is a lot of heat transfer, so much so that the outside surface, even with on the order of $10^9\ \mathrm{Yg}$ of stellar matter piled on top, has to then be at 5800 K - over twice the heat of a tungsten bulb filament - in order to balance it. The reason that surface is cooler than the core is in part because it is a lot bigger than the core surface and also because the matter immediately surrounding the core transfers a sizeable amount of the heat right back in, thus slowing down the rate at which it goes out. Heat "percolates" from core to surface, instead of just rushing out all at once.
Insofar as the relation to fusion is concerned, the answer is that fusion keeps the Sun's core hot. It did not make it hot to begin with: that heat was originally provided by the gravitational contraction of the gas cloud that formed the star in the first place. That's how the core first got to 15 MK, at which point the rate of fusion became enough to sustain it there in an equilibrium by replenishing the heat being lost. (Actually, it was likely cooler. As the Sun has aged, it has steadily grown hotter, for reasons that are a bit complicated to get into in full detail but relate to the depletion of the hydrogen fusion fuel over time causing the star to contract and thus release more gravitational energy.) The surface would still be just as hot and interesting were there no fusion - although if that were the case, the Sun would gradually contract further and eventually become a dense white dwarf, and would have done so long before the present day.
Also, if you are asking about the various complicated phenomena on the surface, i.e. things like the "bubbly" appearance it has, the flares and prominences, sunspots, etc. - these things are all basically the result of the fact that all that hot gas also moves and churns thanks to the temperature gradient and, moreover, due to its heat, it is actually not just "gas" but a plasma, which then adds complicated electromagnetic effects into the mix as it generates its own magnetic fields with which it also interacts.