Exactly. There could not be true / full ownership of hardware.
And yet that's fine for me.
Now about that:
Today you can make sure the source code is truly what you intend, by running Linux on PC and GrapheneOS on Android. You might not have the ability to audit those, but others (like me) do, and are doing so.
Even in that case you can never be sure what a compiler did with the code. You can say: go look at the code of that compiler. But then how can I be sure it's code had been compiled without malicious modifications. And so on.