Don't you think there are too many parts in history books that make you wonder, "How on earth did the historian know this??"

‌While reading Sima Qian's Records of the Grand Historian, I felt something.
After Qin Shi Huang died, Li Si and Zhao Gao conspired together in a secret meeting to install Huhai as the Emperor.
This must have been an ultra-ultra-ultra-ultra-ultra-ultra-ultra-ultra-secret meeting, the highest level of secrecy imaginable.
So how did Sima Qian know about this?
And who leaked it?
Also, when Kuai Tong urges Han Xin to betray him, he uses all sorts of metaphors and historical anecdotes to persuade him.
Where did he hear those words to repeat them?
To be honest, I feel like there are parts of history that are accepted as established facts despite being somewhat dramatized.
No one actually knows if it really happened that way.
We just believe it to be so.