How does stunnel redirect traffic?
It doesn't; the application needs to be configured to point to the tunnel endpoint.
In the case you're referring to, the client would need to be reconfigured to point to the local stunnel listener, which will wrap the connection data and send it to the server according to the stunnel configuration.
There's also "transparent proxy" mode, which involves explicitly sending the traffic to the stunnel process with iptables, but isn't often used.