After the trap ALL drains are combined. Above the trap the homeowner should clean. This article isn't an indictment of plumbing drains but rather a commentary on the sloven nature of most people. Bio film on the surface comes from not cleaning your sink. And yes... I am a plumber.
“This article isn’t an indictment of plumbing drains but rather a commentary on the sloven nature of most people.”
Where in the article does it say that?