With default configurations, does istio-proxy(sidecar) manipulate incoming/outgoing requests from the application container?
Comment posted by David Maze is good and this could be part of an answer:
There are a couple of cases like distributed tracing and mutual TLS that add headers;
These are the two methods by which you can manipulate the headers using istio. Additionally, in the official documentation you can find a simple tutorial with yaml files, where you will learn how to manage headers. It is also displayed how you can create request header operation. However, take into account that this tutorial is based on an outdated version of Istio and you should use the latest supported version for your solution.