Let $K$ be an abstract simplicial complex on the (finite) vertex set $V$. The geometric realization $|K|$ is typically defined (see Spanier's book for instance) as the collection of functions $\alpha:V \to \mathbb{R}$ so that (**a**) the support of each $\alpha$ is a simplex, and (**b**) the sum $\sum_{v \in V}\alpha(v)$ equals $1$. Now each (closed) simplex $\sigma$ is realized as the collection of $\alpha \in K$ so that $\alpha(v) \neq 0$ implies $v \in \sigma$. From this one knows the star of each simplex.

A simplicial approximation of $f:|K| \to |L|$ is a simplicial map $g:K \to L$ so that $f(\text{star }\sigma) \subset \text{star }g(\sigma)$ for each simplex $\sigma \in K$. It is a standard result that the Piecewise Linear map induced by $g$ is homotopy equivalent to $f$

Now consider the case where $K$ is not abstract, but rather $V$ is an open cover of some topological space $X$. So, each simplex corresponds to an actual topological space, i.e., a non-empty intersection of some finite open sets in $X$. Let's call this $X_\sigma$.

My question is this:

What is the relation between $|K|$ and $X$, more specifically between $|\sigma|$ and $X_\sigma$ for each simplex $\sigma \in K$?

Here is some idea of what type of answer I am hoping for:

In the case where $X$ is paracompact and $V$ is a contractible cover, the nerve theorem applies and I know that $X$ and $|K|$ are homotopy equivalent. But is there a more general relationship between these two notions of realization of which the Nerve theorem is a consequence?

Furthermore, is there some functoriality to the nerve theorem? That is, assume you are given contractible covers $U$ and $V$ of $X$ and $Y$ generating the nerves $K$ and $L$. Given a function $f : X \to Y$ and a simplicial map $g:K \to L$, is there some magic analogoue of the star condition like $f(X_\sigma) \subset Y_{g(\sigma)}$ that makes $g$ induce a map homotopy equivalent to the composite $|K| \to X \to Y \to |L|$ where the maps on the edge come from the nerve theorem and the map in the middle is $f$?