TikTok Is Our DJ Now. It’s Playing a Lot of Meghan Trainor.

Meghan Trainor, the pop star, is sitting in an empty marble bathtub, fully clothed and flanked by two of her friends. The three of them begin soulfully crooning an a cappella version of “Made You Look,” the hit single Ms. Trainor released in the fall, nailing every note for about half a minute. Then they stop, look at one another and scream with delight.

TikTok users went wild for that snippet in November, sending it past 100 million views and attracting comments like “I’m certain this is playing at the gates of heaven.” It’s now Ms. Trainor’s most popular video on the platform.

While the performance seemed informal — after all, it was filmed in a bathroom — it was an example of how Ms. Trainor managed to get TikTok down to a science in the past couple of years, reviving her music career and winning her mainstream popularity in a way she hadn’t seen since she released “All About That Bass” in 2014.

When that upbeat, doo-wop body positivity anthem — “every inch of you is perfect from the bottom to the top” — and its pastel-colored music video earwormed its way into public consciousness, Nielsen said it sold 5.8 million copies to become the 2010s’ highest-selling digital song by a female artist. Ms. Trainor won the Grammy for best new artist in 2016.

Now, TikTok is the engine that drives streams on Spotify and influences what’s on the radio and Billboard charts. Popularity there is currency that record labels crave — and are hungry to replicate.

Ms. Trainor now has nearly 18 million followers on TikTok, in large part thanks to “Made You Look,” which inspired a viral dance challenge shortly after it was released in October. For comparison, Taylor Swift, who uses the app sparingly, has 18.9 million and Lil Nas X, one of the platform’s breakout stars, has 29 million.