Game was made in Unity, and multiplayer solution is Photon Pun. Photon provides free servers for online Unity games. Also, there's a really good Blackthornprod video about making online multiplayer games with Unity & Photon, which taught me the basics a while ago. So if you want to get started this is the starting point I'd recommend. Also, online multiplayer is very hard at first, but after making a few multiplayer games, it gets significantly easier so don't give up!