I hope these questions are not too trivial.
Let $I$ be an ideal in $R$. Write $I'\subseteq R[t]$. Then the notion of tensoring $ (R[t]/I')\otimes_{\,\mathbb{C}[t]} \mathbb{C}[t]/\langle t-c \rangle $ is thought to be restricting to the fiber over $t=c$.
On the other hand, considering $\mathbb{R}$, $ \mathbb{R}\otimes_{\,\mathbb{R}}\mathbb{C} $ is thought to be a base extension.
Question 1: So tensoring is not only thought of as a restriction, but it is also thought of as an extension? Why do we need or when do we use base extensions?
Question 2: Geometrically, what are
$\operatorname{Spec}(\mathbb{C}[s]\otimes_{\,\mathbb{Z}}\mathbb{C}[u,v])$?
$\operatorname{Spec}(\mathbb{C}[s]\otimes_{\,\mathbb{R}}\mathbb{C}[u,v])$?
$\operatorname{Spec}(\mathbb{C}[s]\oplus\mathbb{C}[u,v])$?