In the notes Geometric Wave Equations by Stefan Waldmann at page 70 they have

Having a fixed Lorentz metric $g$ on a spacetime manifold $M$ we can now transfer the notions of special relativity, see e.g. 50 , to $(M, g)$. In fact, each tangent space $\left(T_{p} M, g_{p}\right)$ is isometrically isomorphic to Minkowski spacetime $\left(\mathbb{R}^{n}, \eta\right)$ with $\eta=\operatorname{diag}(+1,-1, \ldots,-1)$, by choosing a Lorentz frame: there exist tangent vectors $e_{i} \in T_{p} M$ with $i=1, \ldots, n$ such that $$ g_{p}\left(e_{i}, e_{j}\right)=\eta_{i j}=\pm \delta_{i j} . $$

We say that two manifolds $M$ and $N$ are isometric if for all $v \in T_pM$ and a map $\phi:M\rightarrow N$ such that

$g(v,v)=g'(\phi^*v,\phi^*v)$ where $g$ is a metric in $M$ , $g'$ is a metric in $N$ and $\phi^*$ denotes a pushfoward.

Now the definition of isometry refers to two manifolds, but in the notes they are claiming an isometry between a manifold and a tangent space.

How is this isometry constructed?