Interesting Triangles Phenomenon

Creation of this resource was inspired by a problem that appears in the November 2021 Mathematics Teachers' Journal published by the Association of Mathematics Teachers of New York State (@AMTNYS). This problem was proposed by James M. Parks from SUNY (Potsdam, NY).

Given angle ADB and DBC are supplementary and AB = DC, how can we prove what is shown towards the end of this animation?

Quick (silent) demo