How passive radar works

Funnily enough, I actually did something like this by accident: https://maurycyz.com/projects/fading/

Got a signal from a person walking ~10m away, and that's with a single antenna and no fancy processing. With one antenna, you only really get an (ambiguous) range and range-rate, so you should really use multiple for good results.

Also, while I used a rather expensive receiver, one of those cheap 20$ RTL-SDR dongles would work perfectly fine. It's also possible to modify them to use an external clock and run multiple receivers at the same time. That would allow using multiple antennas for direction finding or expanding the effective bandwidth.

I don't have any specifics, but for FM/TV band stuff, you should be able to pick up the actual signals with a cheap SDR and a good antenna. The software will probably be... more challenging. And i have no idea what the timing resolution will need to be like.

I'm not sure how likely good software is these days. I had a friend doing research on this area and there was pressure from the military: "Oh, that's funny, I detected a plane flying by", "no, you definitely did not, we don't talk about that - back to your actual research plan". Maybe it's more accessible these days, but close to 2 decades ago it involved mathematicians to go beyond basics.