how can the inside matter, composed of normal matter (so no anti-matter), be transformed into photons only?
Firstly, I need to make some disclaimers. We don't have a fully-working quantum gravity (QG) theory, so we don't know exactly what happens to matter when it reaches the core of a black hole. And we can't be certain that Hawking radiation is real without a proper QG theory: Hawking's calculations involve a semi-classical approximation that "bolts on" quantum corrections to the purely classical GR equations, and we need QG to justify that procedure.
However, what goes on inside a black hole isn't actually relevant to the rest of the universe. As I mentioned in a comment, the matter and energy inside the event horizon (EH) cannot affect the outside universe in any way. All events on or inside the EH are in the future light-cone of the observer, and of course events must be in the past light-cone to have an effect on the present.
As I said here, the gravitational field of a black hole is sometimes described as a "fossil field". All matter & energy falling into the black hole modifies the spacetime curvature as it approaches the event horizon. And once it crosses the event horizon it can no longer change the spacetime curvature outside the horizon, so those curvature changes are preserved (until something else comes along to add its own curvature changes).
The same reasoning applies to the electromagnetic charge of the infalling matter and its effect on the electromagnetic field in the vicinity of the BH. It does not apply to the strong or weak nuclear "charges" because those interactions have finite range; only gravitation and electromagnetism are preserved due to their infinite range. This gives rise to the no-hair theorem:
The no-hair theorem states that all black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum. — Misner, Charles W.; Thorne, Kip S.; Wheeler, John Archibald (1973). Gravitation
The result was quickly generalized to the cases of charged or spinning black holes. There is still no rigorous mathematical proof of a general no-hair theorem, and mathematicians refer to it as the no-hair conjecture.
So when particles fall towards a BH they "pump" energy into the spacetime curvature and EM field of the BH. We don't know or care what happens to those particles once they cross the EH because in our frame that's always in the future.
It's not easy to get an intuitive picture of what happens in and around a BH. Our intuitions aren't very good at dealing with curved spacetime. ;) But here's an analogous situation in flat spacetime that may be helpful.
Imagine we set off an H-bomb at location X at noon on Tuesday. If we try to measure the energy of the blast on the previous day, we won't measure anything, no matter how close we get to X, how big the bomb is, or how sensitive our instruments are. The blast energy simply doesn't travel backwards in time.
Similarly, energy from events inside the EH would have to travel backwards in time to affect events outside the EH.
I should mention that the EH is observer-dependent. The "official" EH at the Schwarzschild radius is the horizon of the Schwarzschild observer. That observer is the limiting case of an observer free-falling towards the BH with zero velocity, so they're at an infinite distance. The horizon for any observer free-falling towards a BH is always below them, until they hit the singularity. This relativity of horizon location means that different observers will measure different numbers of particles in the vicinity of the BH, and is ultimately what gives rise to Hawking radiation, as John Rennie explains here. The mathematics used to perform the Hawking radiation calculations involves the Bogoliubov transformation, which is a bit above my paygrade. ;)
In summary, the Hawking radiation is produced from the energy stored in the gravitational & electromagnetic field around the BH, and the types of particles that originally caused those stresses and strains in those fields is irrelevant.
Now, according to the Bekenstein bound, the entropy of a BH is proportional to the surface area of the EH. So the information regarding the particles that formed the BH isn't exactly lost, but it's not exactly accessible either. As far as I know, Hawking radiation is supposed to be perfectly thermal, so we can't decode that information from the Hawking radiation.
BTW, you may enjoy playing with the Hawking radiation calculator. It makes a few approximations, but it's generally pretty good.
