This is quite a detailed question: I'm struggling to understand a few parts of a proof of the following Lemma. I've placed stars ($\bigstar$) where I'd like to draw your attention.
Lemma: Let $P:Y\to X $ be a covering projection and $ f: Z \to X $ a continuous map where $Z$ is a simply connected and locally path connected space. Suppose given base points $x_0, y_0, z_0$ of $X,Y,Z$ with $p(y_0) = x_0 = f(z_0) $. Then there is a unique continuous $ \tilde{f} : Z \to Y$ with $ p \tilde{f} = f$ and $ \tilde{f}(z_0) = y_0$.
The proof, summarised/paraphrased, is as follows:
Given $ z \in Z$, choose a path $u$ from $z_0$ to $z$ in $Z$. Let $ \tilde{u} $ be the unique lifting of $fu$ to a path in $Y$ starting at $y_0$, and define $ \tilde{f}(z) = \tilde{u}(1) $.
Can then show $ \tilde{f} $ is well-defined, and that it's the only possible mapping that could work. It remains to be shown that it is continuous.
To do so, let $ z \in Z$ and let $V$ be an open neighbourhood of $ \tilde{f}(z)$. Without loss of generality ($\bigstar$), we can assume $V$ is of the form $ h^{-1}(U \times {d}) $, where $U$ us an evenly covered neighbourhood of $ p\tilde{f}(z) = f(z)$ and $ h: p^{-1}(U) \to U \times D$ is a homeomorphism with $D$ discrete.
$f^{-1}(U)$ is an open neighbourhood of $z$, so contains a path connected neighbourhood $W$. For any z' \in W, we can choose a path from $z_0$ to z' of the form $u.v$ where $u$ is a path from $z_0$ to $z$ and $v$ takes values in $W$. Now $fv$ takes values in $U$, which is evenly covered by $p$. So its lifting to a path in $Y$ starting at $\tilde{f}(z)$ must take values in $V \ (\bigstar \bigstar)$, and in particular \tilde{f}(z') = \tilde{(u.v)}(1) \in V. So $W \subseteq \tilde{f}^{-1}(V) $ and hence $\tilde{f}$ is continuous.
My confusions are as follows:
$ \bigstar $: Is the following reasoning as to why this does not lose generality correct? Given any $y = \tilde{f}(z)$, consider $ p(y) $. We can find an evenly covered neighbourhood $U$ containing $p(y)$. Then there is a homeomorphism $ h : p^{-1} (U) \to U \times D$ where $D$ is a discrete space. Then $ (p(y), d) \in U \times \{d\} $ for any $ d \in D$ and $ h^{-1} (p(y),d) \subseteq h^{-1} (U \times \{d\})$. But $ h^{-1}(p(y),d) = p^{-1} p(y) $ which contains $y$.
$ \bigstar \bigstar $: I'm confused with this bit. Intuitively, $ p^{-1}(U)$ 'looks like' lots of copies of $U$, and $V$ is one of those copies. But I can't turn this into a definitive reason why $ \tilde{v} $ (i.e. the lifting of $fv$) must be entirely contained within $V$. On a different note, am I correct in saying $ V \cong U $? I'm convinced there's a simple set-theoretic explanation, but it is eluding me.
Any help will be greatly appreciated - I understand that this is a long post.
Thank you.