It's not possible to replicate the CDN, or DNS, or the message queue if it's hosted, or the database of it's big enough, or etc. etc. The differences between what you're able to create in a staging environment and what exists in prod are significant enough that testing against the staging environment doesn't build much more confidence than testing with mocks.
Testing in prod means accepting this reality, avoiding the largely unproductive toil of building and maintaining a staging env, and making the production system observable, resilient, and operationally agile enough that you can deploy changes that can trigger unpredictable and emergent system behaviors while managing risk appropriately.
Testing in prod means accepting this reality, avoiding the largely unproductive toil of building and maintaining a staging env, and making the production system observable, resilient, and operationally agile enough that you can deploy changes that can trigger unpredictable and emergent system behaviors while managing risk appropriately.