How do polyurethane varnishes cure after only the solvent evaporates?

I believe that polyurethane varnishes have as components isocyanates & polyols that react to form the polyurethane portion of the cured varnish. How is it that this doesn't happen spontaneously while the varnish is still in the can & only after it's been left for the solvent (water or organic solvent) to evaporate? What's the chemistry here?

Does the polyol only become a polyol after the solvent have evaporated? If so, again, what's the chemistry there?

How do polyurethane varnishes cure after only the solvent evaporates?

I believe that polyurethane varnishes have as components isocyanates & polyols that react to form the polyurethane portion of the cured varnish. How is it that this doesn't happen spontaneously while the varnish is still in the can & only after it's been left for the solvent (water or organic solvent) to evaporate? What's the chemistry there?

Does the polyol only become a polyol after the solvent have evaporated? If so, again, what's the chemistry there?