And even if you fabricate your own hardware, you're still trusting the open source designs you download from the internet. Being able to audit them is much better than not being able to see them, but it's still very possible to have some incredibly subtle backdoor or vulnerability hiding in plain sight. This happens all the the time in complex systems. There is zero chance that you can design a functioning and useful chip and OS, from scratch without trusting something else along the way. That goes all the way to compilers, toolchains etc... And even if you could, chances are you've created hundreds of accidental security vulnerabilities that can be exploited. Zero trust is a fantasy.