How do I turn my game into multiplayer?

I am trying to make an online multiplayer platform shooter and I've been confused how does the Multiplayer built-in plugin, but even after buying an example I am still very confused about it. I am willing to share my project if someone knows how to make it multiplayer.

I think the best thing you can do is try to understand how multiplayer works in a very simple project like 2 cubes moving in 8 directions and see if you can sync them across both screens and then know how to get in and out of a room , because converting a game to multiplayer is not an easy task.

ok thanks