Variance of a random variable obtaining from a linear transformation

Edit: I needed to revise this question as it was suggested.

Suppose there are $$N$$ realizations of Gaussian process denoted as the vectors $$z_{j} \in \mathbb{R}^{n}$$ for $$j = 1, \ldots, N$$. Let $$y$$ be a random variable such that $$y = \sum_{j=1}^{N}(Bz_{j})[i]$$ where $$B$$ is a unitary matrix. What is the variance of $$y^{2}$$?

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