How do we show that the polynomial time reduction of one problem to another has been done in polynomial time?
The reduction described in the mentioned post, from SAT to the halting problem, performs the following mapping:

$$\varphi\mapsto \big(\langle M\rangle,\varphi \big)$$

where $\varphi$ is a CNF formula and $M$ is the Turing machine which given a CNF $\psi$ as input, iterates over all the possible assignments to the variables of $\psi$, and enters a loop iff no satisfying assignment was found.

The machine $M$ does not depend on the input of the reduction, so it has some constant size description independent of the input $\varphi$. This means that you can output $\langle M \rangle$ in polynomial (even constant) time relative to the size of $\varphi$, hence the reduction is computable in polynomial time.

Note that you only need to show that you can generate $\langle M \rangle$ in polynomial time, which does not necessarily mean it has to have a constant size. Suppose e.g. that we want to show that $L=\{\langle M\rangle | \text{M halts on input $\epsilon$}\}$ is NP hard. In that case, the reduction $\varphi\mapsto \langle M_{\varphi}\rangle$ does the trick, where $M_{\varphi}$ is the machine which ignores its input, iterates over all possible assignments for $\varphi$, and enters a loop if no satisfying assignment was found. $M_{\varphi}$ does not have a constant size description, since you have to hardcode $\varphi$ into it. However, you can still generate $\langle M_{\varphi}\rangle$ in polynomial time, which makes the reduction polynomial.