If a monster makes an improvised weapon or unarmed attack, is it just adding strength?

I’m actually not sure how 5E works. If a monster makes an improvised weapon or unarmed attack, is it just adding strength? Or does it retain most of its damage dice and/or change die type? Unarmed strikes are 1 bludgeoning plus Strength modifier, no matter who makes one.

— Dan Dillon 👥 (@Dan_Dillon_1) July 21, 2019

Yup.

— Dan Dillon 👥 (@Dan_Dillon_1) July 21, 2019