MetrixVR is a multiplayer online game, where you can build things, talk with people, and much more.
The game have many gamemodes, from "sandbox" to some gamemodes with survival or exploration goals.

How to Play Gamemodes ?

There is two ways to play: Using the main server (Hub), or on some self-hosted servers (MVRDS / MetrixVR Dedicated Server).
The hub allow you to play with a limited sandbox game. It also make you able to meet other community members, or just to wait for a private server to be online.
The self-hosted servers allow you to play any gamemode, as long as you respect the rules created by the server master.
Theses self-hosted servers are hosted by community members or some third-party organisation, and allow the host to configure his server as he or his community wants.

How to create custom game content ? (Objets, Models, UIs, Gamemodes, ...)

To create some content, you have to use the game developpement kit, called "UVRSDK", and Unity 2018.3.0f2.
If you already used to create some avatars or worlds using VRChat, this should be really easy for you to use it.