While the video is long he does a pretty good job of explaining how radar works. I was a radar tech in the USAF and some of it went over my head. Anyway the simple is the satellites shoot a signal down and it reelects back up to the antenna. All the satellites create a huge antenna array that can create 3d images of the ground in real time. Couple that with other types of data and we could be found just about anywhere at anytime.

There are plenty of survey satellites anyway, the only reason you need so many Starlink satellites is because they need to be low for latency and therefore need a huge number for coverage.

Wi-Fi 2.4-5 ghz is a decent radar spectrum.... These are technically microwaves. Radar often uses radio waves. Where one ends exactly and the other begins is arguable. They are neighbors in the electromagnetic spectrum.

Depending on specific frequencies that you pick within their ranges you might be able to penetrate through certain objects, detect certain things based on absorption (water, bodies), etc.

A satellite constellation is a network of satellites. They aren’t geostationary, but can cover the whole globe because there’s so many of them. When a connection to one drops, it hands off to the next one. GPS and Starlink are examples.

Radar satellites use radar to image the earth, so called SAR. It can be used to observe the ground even with cloud cover.

A radar constellation combines this.