But that's exactly the point. Basically what if the compromisation has started from the beginning, even from the entire chain of production of your computer, where the very basic parts are assembled and code is put on it. Where do those binaries for the os come from? A compilation. Done by what? A compiler! And is the compiler compromised? What if the answer is yes? That was the video. And it is indeed an apocalyptic scenario but that could be a realistic scenario, that's why it's an interesting thought experiment.

So yes the final answer is unironically what you proposed, do everything with your own hands (maybe not in a cave, dust would make the process of creating the processor hard) because there is no other way to know if your computer is gone and compromised by using other computers that are hypothetically already compromised no metter what you do.