On the PHB p. 72 it just says:
Beginning when you choose this archetype at 3rd level, your weapon attacks score a critical hit on a roll of 19 or 20.
So my assumption is that on attacks made by Booming Blade or Divine Smite, the crit still applies to the regular attack's effects, but does the critical hit then effect with the Booming Blade's on hit Thunder damage, or the Smite's Radiant damage? What about with the bonus action smites? Like Thunderous Smite, does it affect those as well?
Basically, my main question is, does Improved Critical basically just boost your critical range, or does it specifically only improve the critical of regular weapon attacks from the attack action?