Qubits that are prepared separately combine by the *tensor product*: two qubits each in (|0⟩ + |1⟩)/√2 make

(|00⟩ + |01⟩ + |10⟩ + |11⟩)/2,

four outcomes with amplitude 1/2 each, probability 1/4 each. With n qubits there are 2ⁿ amplitudes. That count is why a classical computer can follow a few dozen qubits exactly and not a few hundred, and why the engine picks its method by the program's shape: some circuits need far less than every amplitude.

A gate on a register's name applies it to each qubit: `h q;` is `h q[0]; h q[1]; …` for every qubit of `q`. `wai-quantum plan` shows the method the engine would pick for a program and why, without running it.