How to make a Game (Multi User Environment) Like SecondLife and IMVU

Hi Guys,
Please help me outs! i had searching this for months but cannot find anything related to it!
All I get is UNET, Photon, Forge Sample Project.
I Had Watched a many tutorials in youtube related to it but cannot find anything!

I like this concept of UNet but when i create a host in 1 Build App and connect to another 2 Build App. and when i close the 1Build App the 2 build App disconnect. I like to know how to connect to the server till there is a single player!

I know its difficult...

How to make a Game (Multi User Environment) Like SecondLife and IMVU