Welcome to the forum, Mirko 🙂
If you finished the regular installation process on a Windows server, REI3 should be running in the background (as a Windows service). To see if that is the case, you can open a browser on that Windows server and go to "https://localhost" - you should then see the login page of REI3 (after a message about an untrusted certificate).
On Windows, you can view all system services by opening a run command line (Windows-key + R on the keyboard) and then typing
services.msc. This will give you a console that displays all running services on this Windows machine. The REI3 service will be visible if installed.
If you need detailed instructions on how to install/configure REI3, the admin documentation should cover that:
But if you have any issues or questions, do not hesitate to ask 🙂