why the photon concept is rarely mentioned in talks about the radio part of the EM spectrum?
A quick calculation shows that the photon concept is irrelevant for typical RF applications. Suppose that we have a detector that can detect some minimum power, $P$, operating at a frequency, $\nu$. The minimum number of photons it can detect is then $$N_\text{min}=\frac{P}{h\nu^2}$$ For an ultra-sensitive detector that can detect $1\text{ pW}$ at $100\text{ MHz}$ that works out to $N_\text{min}=150000$ photons as a minimum detectable threshold for a very sensitive RF detector. At those large numbers you can reliably replace any quantum mechanical operators with their expected value and use the classical operators instead.
So what is going on at the reception site that makes it seeing these interferences?
Continuing with the above numbers. Suppose that we have a phased array detector with 150 elements, and suppose that we are trying to detect a minimum-power signal. Then, even for this minimum detectable signal we expect 1000 photons per element in the array. That is already enough to reasonably use a classical approximation, but you could use a full QM approach if desired.
Photons are not localized, but have a wave function that spreads out over space. The photon wave function is governed by Maxwell’s equations, and is basically $\Psi=E+i B$. The square of this wave function gives the probability density of a photon interacting with each element of the detector. Since each detected photon gives a voltage in the element, this simply produces a signal in each element approximately as we would expect classically.
Although the phased array detector expects an average of 1000 photons per element, the actual amount will be highly non-random. Some elements may have 0 and some may have 2000, for example. A phase sensitive detector expects that, and further knows that if a specific element receives 2000 then a specific different element will receive 0. The detected signals are then amplified so that they are fully in the classical regime and then added in a phase sensitive manner. This means that if the amplified element signals match the expected pattern then the phase sensitive detector registers a signal, and if the amplified element signals do not match the expected pattern then no signal is registered. Because this involves for example hundreds of elements each detecting a thousand photons on average it is well within the classical regime. Photons bring no benefit to the analysis, even though they are there.
Edit: As mentioned above, the square of the wave function $\Psi$ gives the probability density of detecting a photon at that location. $\Psi$ behaves linearly, loosely meaning that if you have two valid wavefunctions you can add them together to get a third wavefunction which is also valid. So if you have a wavefunction for one photon and a wavefunction for another photon then the wavefunction of the system of both photons is their sum.
The probability of detecting a photon at a location is then given by the square of the sum of the wavefunctions. In locations where the two wavefunctions are out of phase they interfere and lead to a reduced magnitude. This therefore reduces the probability of detecting a photon at that location. So photon counts and interference are two ways of saying the same thing. In regions where the wavefunctions are in phase they add constructively and increase the photon count and in regions where they are out of phase they add destructively and decrease the photon count.
