Radar uses (massless) photons, as all optical phenomena on the electromagnetic spectrum involve. Radar systems emit radar wave/photons at certain frequencies, and those photons can interact with atoms in material (i.e., non-optical) objects, causing electrons in those atoms to increase to higher levels of energy and then to re-emit photons as they revert to lower levels. Those photons are what we call "reflections", just as light from a flashlight is reflected from material objects, at frequencies our eyes can detect. That's how radar is supposed to work, and generally does work: by shining radar beams and observing the reflections.

However, like all optical phenomena, radar waves and reflections are subject to scatter (including multiple reflections), refraction, and interference from other electromagnetic waves. To understand that, it is helpful to read the Wikipedia article on Radar (https://en.wikipedia.org/wiki/Radar#Limiting_factors). See the section on Clutter in particular.

Although that article doesn't mention spoofing, it's possible that the US military has developed radar spoofing techniques to make objects (decoys) appear to be detected by radar even when there are no such objects. That would be very useful against the radar equipment of enemies. Some reports of UAP sightings on radar may be spoofed decoys.

Of course there have been many kinds of reports of UFOs and UAP sightings, and many different explanations for those. Most don't involve radar, but some involve infrared detection, for example. There's a good video on YouTube that explains, among other things, how some reports of UAPs can result from misinterpreted infrared images: https://www.youtube.com/watch?v=VCH7BWGpl5s

In short, there's not always a direct connection between what we "see" and where the photons originated. Both our eyes and our instruments can be deceived.