  Let $(G,X)$ be a PEL-Shimura datum of type AC in Kottwitz's classification.
Assume $G_{\mathbf{Q}_p}$ is unramified. We show that the good reduction locus
of the infinite $p$-level Shimura variety attached to this datum, considered as
a diamond, can be described as the fiber product of a certain v-stack (which we
call ``Igusa stack") with a Schubert cell of the corresponding
$B_{dR}^+$-affine Grassmannian, over the stack of $G_{\mathbf{Q}_p}$-torsors on
the Fargues-Fontaine curve. We also construct a minimal compactification of the
Igusa stack and show that this fiber product structure extends to the minimal
compactification of the Shimura variety. When the Schubert cell of the affine
Grassmannian is replaced by a bounded substack of $\mathcal{G}$-shtukas, where
$\mathcal{G}$ is a reductive model of $G_{\mathbf{Q}_p}$ over $\mathbf{Z}_p$,
we show that this fiber product recovers the integral model of the Shimura
variety. This result on integral models, if specialized to a Newton polygon
stratum, recovers the fiber product formula of Mantovan. Similar fiber product
structures are conjectured by Scholze to exist on general Shimura varieties.

Number Theory;Algebraic Geometry