BarryK can give you the N'th degree explanation but simply put, Poly means "Many" so polyurethan's have more solids. Such as Polyurethane build primers like FeatherFill 2 by Evercoat. Like sprayable plastic filler.
Clear over/mixed with a single stage paint is as old as clear coat. It gives a nicer finish and more depth as the clear is a magnifier.
The downside is,Those jobs are a royal pain to repair.