What do these million dollar salary employees at Gmail do?

Three things, not all of which any specific employee does: 1. Fix security issues 2. Create “features” in order to seem useful that the world was better without 3. Rest upon laurels of gmail from 15 years ago

Make Google multiple millions by improving ad delivery and conversion within Gmail. Probably by also helping Google land big corporate or public contracts, but last I checked most of the money was made via ads in the free tier of GMail.