Mr. Yu’s claims, which describe how ByteDance operated five years ago, are surfacing as TikTok faces intense national scrutiny over its relationship with its parent company and China’s potential influence on the platform. The video app, which is used by more than 150 million Americans, has become hugely popular for memes and entertainment. But lawmakers and U.S. officials are concerned that the app is passing sensitive information about Americans to Beijing.

In March, a congressional committee grilled TikTok’s chief executive, Shou Chew, about the app’s Chinese ownership. Christopher Wray, the director of the Federal Bureau of Investigation, recently said that TikTok “screams out with national security concerns.” More than two dozen states have banned TikTok from government devices since November.

TikTok did not immediately respond to a request for comment.

In his complaint, Mr. Yu, 36, said that as TikTok sought to attract users in its early days, ByteDance engineers copied videos and posts from Snapchat and Instagram without permission and then posted them to the app. He also claimed that ByteDance “systematically created fabricated users” — essentially an army of bots — to boost engagement numbers, a practice that Mr. Yu said he flagged to his superiors.

Mr. Yu says he raised these concerns with Zhu Wenjia, who was in charge of the TikTok algorithm, but that Mr. Zhu was “dismissive” and remarked that it was “not a big deal.”

Mr. Yu, who spent part of his ByteDance tenure working in its China offices, said he also witnessed engineers for Douyin, the Chinese version of TikTok, tweak the algorithm to elevate content that expressed hatred for Japan. In an interview, he said that the promotion of anti-Japanese sentiments, which would make it more prominent for users, was done without hesitation.