Is Everything Quantum Computers?

A quantum computer is one whose computations rely on distinctively quantum effects, mostly interference and entanglement. There's a difference between this computer that we're using to communicate here and the quantum computer that several companies are currently trying to build. And even the human brain, your contention is that it's a classical computing system. We don't know exactly how it works, and some people do think it may rely on quantum effects, in which case it is a quantum computer. But I don't think so. For various reasons, it seems very implausible to me that it would be one.