There is a quotation below:
Let $\{e_{i,j}\}_{1\leq i, j\leq n}$ be a system of matrix units fro $M_{n}(\mathbb{C})$ and consider $$\sum\limits_{i,j=1}^{n}e_{j, i}\otimes e_{j, i}.$$
A straightforward computation shows that this matrix is equal to $nP$ where $P$ is the one-dimensional projection onto the span of the vector $$v=\sum\limits_{k=1}^{n}\delta_{k} \otimes \delta_{k}.$$
The $\{\delta_{k}\}$ is an orthonormal basis of $\mathbb{C}^{n}.$
My question is: how to comprehend the "equal to" here? And how to verify it?