a puzzle to solve...how does a battery operated flashlight interfere with radio signal??

Depends on the flashlight. Some flashlights have a switching power supply between the batteries and whatever they use to make light (a bunch of LEDs or a fluorescent tube) to increase the voltage and switching supplies can be electrically noisy. Radio signals are small so it doesn't take much radiated energy to interfere.

If it's an old-fashioned flashlight with only batteries and an incandescent bulb, I have no idea.

Maybe when a torch is switched on it releases a weak electromagnetic field. I'm just guessing.

You did provide enough details, but my guess is that you scenario involves a TV or radio in the basement, and an LED flashlight.

If this is the case, the problem is that the LED flashlight is using non-EMC (EletroMagnetic Compliant) LED bulb.

The solution is to switch to a flashlight with LEDs that are EMC certified. This could also affect the operation of a computer as well.

I do not think it can.

I have no idea, something to do with the Ionosphere?

https://en.m.wikipedia.org/wiki/Electromagnetic_interference