Sarah Tang did not set out to be internet famous.

Ms. Tang, 28, does not sing, dance or share outfit-of-the-day videos. She does not post about her personal life, offer career or dating advice, or share finance tips. She does not have an agent or a manager, and she is not flying off to brand-sponsored retreats in coastal cities. Her TikTok following is modest.

Still, Ms. Tang is often recognized, particularly on the J train in New York City, for her appearances in TikTok video ads by buzzy brands targeting Gen Z and millennials.

“I started feeling really anxious about things like falling asleep on the subway or just like looking really grumpy,” Ms. Tang said. “I’m not, like, anonymous anymore.”

Ms. Tang is what some in the fashion and lifestyle industries call a brand or in-house content creator. Unlike influencers, who are typically paid to promote products on their own channels, Ms. Tang’s job is to produce short-form videos that emulate influencer-style content like room tours, vlogs and get-ready-with-me snippets on brands’ own social accounts. Her videos can generate thousands of views.