What's the difference between hemp and marijuana legally?

Legally the distinction is a number rather than a plant. Cannabis below the statutory THC concentration is hemp and lawful. Above it, the same plant is marijuana and controlled.

The consequence is that the line is invisible without laboratory testing. Hemp flower and marijuana look alike, smell alike, and burn alike. No officer can distinguish them at a roadside, and neither can a drug dog.

That indistinguishability is precisely why Florida's appellate courts have been dismantling the plain smell doctrine — an odor that could equally indicate a legal substance can't by itself make criminality apparent.