Important questions about professional multiplayer games

I have some questions that cause me crazy about the multiplayer games
Suppose that I have created a game and after designing and programming it, I would like to add the Multiplayer feature _________________________________________________________________________________________________
Here are the problems …
1- How can we make the Multiplayer system? (I know that I have to learn the blueprint Multiplayer course )
But how can we actually do a big Multiplayer system and servers

And how to know the number of players and their own information and how to let players register with their own emails and how to create a control panel to manage the players and know the number of players in the game and the databases ?
I really looked everywhere for these questions and I wondered if there are sites offering this service or should I do it personally

I really hope that my questions will be answered … finally i would like to thank all of this big community

To make a single player game into a multiplayer game is not a simple task and it is often easier to start over on any logic related to multiplayer.

Handling your own servers is a bad idea unless if you are a professional.

Handling players personal information like e-mail is an even worse idea especially if you are providing services to EU citizens as they are protected by strict Data Protection Regulations (GDPR). It is better to have other capable providers handle that like Steam.

I am not trying to frighten you from making a multiplayer game but it is a bit of a challenge if you haven’t done it before so you should hire a professional or start small.

You don’t “add multiplayer later”…
Adding multiplayer equals to building a whole new game.