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From what I heard about the SMART-L [1] radar around 1990, it could spot reflective objects the size of tennis balls at a distance of about 100Km.

I guess with modern electronics and DSP, a much smaller system must be able to detect drones with ease.

[1] https://en.wikipedia.org/wiki/SMART-L

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The problem isn't detecting small objects, it's that at this scale, there's a ton of noise. Distinguishing between noise, foliage, birds, and a tiny FPV drone, is really finding a needle in a haystack. Radar systems have been simply ignoring that level of noise at low altitude, sticking to fast moving small objects (artillery shells, rockets) or generally bigger objects (planes).

What if you cover the drone with non-reflective material? Or does it care about the reflective nature of the core of the object rather than just the surface?

Frequency is relative to the size of the object you want to detect and the size of the wave. Higher frequencies detect smaller things and can discern between them. Go too far and you detect bugs. Too low and you only see barn doors.

Spinning blades are a dead giveaway for radars. Certain things reduce radar signature but the interesting part now is electronic countermeasures.

A radar can produce a fuzzball graphic to the viewer of the data, speed, distance, direction of movement, etc.

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