Mr. Mohan inherits YouTube during a challenging period. The company is contending with a slowdown in digital advertising that has halted its growth, and it faces increasing competition from TikTok, a Chinese-owned short video service.
YouTube added its own short-video feature, Shorts, to compete, and it has tried to become a destination for more premium content by offering cable television programs and the N.F.L. Sunday Ticket, a package of weekly football games. Mr. Pichai said this month that Shorts was now averaging 50 billion daily views. The service has also begun running ads and sharing revenue with creators on the platform, an effort to lure influencers away from TikTok.
YouTube has recently lost several prominent executives, including Robert Kyncl, who was chief business officer before leaving at the start of the year to become the chief executive of Warner Music Group, and Martin Kon, YouTube’s business finance officer who departed for the A.I. start-up Cohere.
“I have never in my career met a C.E.O. like Susan,” Mr. Kon, who is now Cohere’s president and chief operating officer, said in an interview. “She was able to go back and forth with engineers, with data scientists, with partnerships people, with marketing people on the details of what they’re doing.”
The bulk of YouTube’s revenue comes from ads that run before, during and alongside videos. In the third quarter, the company’s ad sales declined 1.9 percent from a year earlier. Last quarter, revenue dropped 8 percent, to $7.96 billion.
The platform has long been keen to diversify its business and has prioritized efforts to expand its content subscriptions. YouTube said in November that it had 80 million paid subscribers for its music and premium services, a jump of 30 million from the previous year.
The platform also offers YouTube TV, a bundle of streaming television channels to replace cable packages. YouTube TV has more than five million subscribers, the company has said. YouTube does not disclose the revenue it generates from subscriptions.