Answer
By default, Application CRs live in the argocd namespace (where ArgoCD is installed). Newer versions support apps in other namespaces (apps-in-any-namespace) with configuration, but the classic model keeps Application resources in the argocd control-plane namespace while the workloads they deploy live in their target namespaces.
Real-world example The guestbook Application object sits in argocd, but it deploys the guestbook workloads into the guestbook namespace.