TV and radio stations can be used for passive radar. Passive radar is when you use existing radio sources as you illumination source and your radar system is listen-only. Stealth aircraft tend to be more visible on passive radar since passive is usually a longer wavelength that the aircraft was not designed for. You can even DIY a passive radar.

https://spectrum.ieee.org/passive-radar-with-sdr

You could use a focused receiver to look for TV signals being emitted from an object thousands of feet in the air over (say) the ocean off the coast of California. Since you know there is no TV station antenna floating in the sky out there you know you are getting reflected rf from a plane.

Your receiver is "passive" - it doesn't emit a signal - so the attacking enemy has to find it visually. If the enemy bomb all the TV antennas then you need to emit your own signal to generate the reflection.

The other way around, it's easy to track these with passive radar which is why all radio emitters get bombed.

Nothing. That's the point. You don't need active radar with the right mix of broadcast stations and math tricks.