Adding a diagonal matrix to a product of transpose of a matrix and itself is always invertible

I assume we're working over $\Bbb R$, the real numbers system.

Since $\mathbf A$ is an $\mathbf n \times \mathbf d$ matrix, $\mathbf A^{\mathbf T} \mathbf A$ is a $\mathbf d \times \mathbf d$ matrix; thus the context indicates that $\text{size}(\mathbf D) = \mathbf d$. We may set

$\mathbf D = \text{diag}(\mu_1, \mu_2, \ldots, \mu_{\mathbf d}), \tag 0$

where $\mu_i > 0$, $1 \le i \le \mathbf d$.

Let

$0 \ne \mathbf x = (x_1, x_2, \ldots, x_{\mathbf d})^{\mathbf T} \in \Bbb R^{\mathbf d}; \tag 1$

then if $\langle \cdot, \cdot \rangle$ is the usual inner product on $\Bbb R^{\mathbf d}$, we have

$\langle \mathbf x, \mathbf A^{\mathbf T} \mathbf A \mathbf x \rangle = \langle \mathbf A \mathbf x, \mathbf A \mathbf x \rangle \ge 0; \tag 2$

furthermore, since the matrix $\mathbf D$ has only positive elements along is diagonal and zeroes elsewhere, we also have

$\langle \mathbf x, \mathbf D \mathbf x \rangle = \displaystyle \sum_1^{\mathbf d} \mu_i x_i^2 > 0; \tag 3$

then

$\langle x, (\mathbf D + \mathbf A^{\mathbf T} \mathbf A ) \mathbf x \rangle = \langle \mathbf x, \mathbf D \mathbf x \rangle + \langle \mathbf x, \mathbf A^{\mathbf T} \mathbf A \mathbf x \rangle > 0 \tag 4$

as well. By (4), $\mathbf D + \mathbf A^{\mathbf T} \mathbf A$ is positive definite; we further see that (4) precludes the possibility that

$(\mathbf D + \mathbf A^{\mathbf T}\mathbf A) \mathbf x = 0 \tag 5$

for any $\mathbf x \in \Bbb R^{\mathbf d}$; thus $\mathbf D + \mathbf A^{\mathbf T}\mathbf A$ is nonsingular, hence invertible, no matter what the values of $\mathbf d, \mathbf n > 0$ may be; in other words, $\mathbf D + \mathbf A^{\mathbf T}\mathbf A$ is of full rank $\mathbf d$.

Indeed: if $D$ a diagonal matrix with non-zero diagonal entries, then $D + A^TA$ will necessarily be invertible.

A succinct proof is as follows: we begin by noting that $A^TA$ is (symmetric and) positive semidefinite, and $D$ is (symmetric and) strictly positive definite. As such, $D + A^TA$ will also be strictly positive definite, and hence invertible.