Court records, mistakenly made public, tell a story about the birth of ByteDance, its bumpy road to success and the role of the Republican megadonor Jeff Yass's firm.

In 2009, long before Jeff Yass became a Republican megadonor, his firm, Susquehanna International Group, invested in a Chinese real estate start-up that boasted a sophisticated search algorithm.

The records also show that Mr. Yass's firm was more deeply involved in TikTok's genesis than previously known. It has been widely reported in The New York Times and elsewhere that Susquehanna owns roughly 15 percent of ByteDance, but the documents make clear that the firm was no passive investor. It nurtured Mr. Zhang's career and signed off on the idea for the company.