You were trying to put Apache on another port in another post. I have no idea what you’re actually trying to do. We have a major XY problem here.
You want to run an application, right? It needs an application server to run, correct?
If so, then you need to start your application server on that port. I believe you were starting gunicorn in another post, so this is a Python application. And, I believe you had it working via http but couldn’t get it working with https (which is fine for a local application).
If that’s so, you should have it listen only on the localhost address (127.0.0.1) on the port of your choosing (8000, to match the proxy config in Apache) and not on a public address or 0.0.0.0.
Then, Apache will provide the https connection on the usual port of 443 and proxy to the local port of 8000.
Your browser does not connect to port 8000 in this scenario. For one thing, it’s not https (and it doesn’t need to be for local connections), for another, it’s problematic to require your users connect to a weird port…they may be blocked by firewalls on their network. Ports 80 and 443 will probably not be blocked.