Why radiowave cannot pass through steel mesh？
I can provide an easier answer, though I suppose it will just make you ask more of them.

In a metal, you can imagine that there is a veritable ocean of freely mobile (so-called conduction band) electrons available in the metal. (It's why it conducts so well.) Simplified, you can almost imagine these as a gas, in fact.

In a conductive mesh (or a microwave oven with holes perforating a conductive sheet), the electrons respond almost immediately to the oscillations that occur when an EM wave is felt by them. They will push on each other while responding to the EM wave. The electrons themselves don't have to travel all the way around, though. All that has to happen is that they are able to adjust themselves fast enough in response to the EM wave. And that can happen at almost the speed of light in the metal. (I'm not talking about the actual drift velocity of one particular electron, but their responses amongst each other relative to the EM wave propagation.)

Note that a long-wavelength EM wave only undulates slowly at any physical point. So there is lots more time for the electrons to redistribute themselves in response to long wavelengths than for short wavelengths. So larger holes will allow short wavelengths to pass, because the electron responses in the metal to the short EM wave "signal" simply can't move rapidly enough around the hole to completely counter that EM wave. But with long enough wavelengths, they can. This is why short wavelengths can pass (at least partially.) The result is that the EM wave is reflected away rather than passing through.

Yet another way to think about it is that the electrons accelerate in response to the EM wave; their acceleration itself produces a magnetic field that joins with their electric field to produce EM radiation; and the phase is such that the transmitted wave is cancelled out. (That is, if the impinging EM wave is undulating slowly enough, of course.)

There are other effects (such as the skin effect that limits the participating electrons.) But that's the gist of it.

P.S. The above effects do NOT apply to water waves hitting a wall with a slot cut in it. There is NO similarity. So you cannot use water waves as an analog for understanding the situation with electrons and EM waves.