I watched a video on YouTube that recently, in which a Chinese mercenary spoke to an influential YouTuber.

I'm guessing my correction is what you mean, but if I take your originally text literally, it could mean that you watched a video that DESCRIBED how a Chinese mercenary spoke to someone on YouTube, but did not actually SHOW him speaking.

If you would add a Chinese version of your text, it would help me to be sure that I'm correcting your text correctly.

The primary reason (why) he fights for Russia is his passport prevents him from entering Ukraine, otherwise he might join the Ukraine army.

"Why" is unnecessary. Many native English speakers say "the reason why" in casual speech but rarely write it.

I am surprised to hear that he thinks many other Chinese on the Russian side also have the same excusereason.

"Excuse" would imply that they don't want to fight on the Ukrainian side, and they point to the passport issue as a convenient justification for fighting on the Russian side.

I watched a video on YouTube that recently, in which a Chinese mercenary spoke to an influential YouTuber.

The primary reason why he fights for Russia is because his passport prevents him from entering Ukraine, otherwise he might have joined the Ukraine army.

I am surprised to hear that he thinks many other Chinese fighting on the Russian side also have the same excusereason.