If I’m not mistaken the EMP wave is really just a part of the high intensity wave of photons of various frequencies emitted by the explosion, which also includes the so-called “bright flash”.
Some of those photons will have wavelengths that put them in the radio part of the spectrum so they can transverse materials which are not transparent to visible light frequency photons and have the right wavelengths to induce strong electrical currents in electronic circuits and even integrated circuits (which is what burns them) - depending on the length of a conductive line of material there often is a perfect radiowave wavelength to induce a current in it (though I confess that over the years I forgot the formulas to calculate this stuff)
I’m not a Nuclear Physicist but I have 1 year of University level Physics training and an EE degree (though focused on digital systems rather than telecomms).
Anarch157a