We have exploded thousands of nuclear weapons below and above ground. Many were very high-altitude. EMP effects are real, but hysteria exaggerates EMT threat. One 10-50kt nuke isn’t going to send us back to the 19th century.
Agree, no 50kt nuke as such. But if they add a depleted-uranium casing, with help from the Russians or Chinese, that significantly improves gamma output, to maximize the Compton effect, and pop it off high enough above the atmosphere, yeah, it could blow every ultra-high-voltage transformer on the East Coast. And America only has one manufacturer left. And they have a backlog. It would be MONTHS without power. Yes, even with a 50 kt nuke.
It's the geomagnetic currents from the Compton effect electrons interacting with Earth's magnetic field that does it. With that level of yield, your iPhone would be just fine. The transformers that couple to the power generating stations, though, would be toast.