Randy, some misinformation may have been presented to you. Not many basecoats are actually Urethane, well not in the sense you think of them as urethane. In my experience, all URETHANE materials are catalyzed before spraying. Now that being said, there are some basecoat systems that react to the catalyst in the clear when you spray it, so they are in a form Urethane.
as for clears, Yes all Urethane clears are by definition catalysed, this is because to become Urethane,k thy need to cross link when they cure, However some of them are catalyzed by chemicals other than something you add to them. This is to say that there are Urethanes that are single pack in a spray can and they catalyze by something present in the air as I understand it. I dont have a lot of experience with those, for our purposes, I would say that any clear Urethane will be mixed with a catalyst before spraying.