Multiplayer is a common feature in games nowadays, but its technical implementation is not always easy. Although Unreal Engine has inbuilt multiplayer features, and it is quite easy to adapt in most simplest cases. However, when your game grows in the number of players and the size of game world and mechanics, you really have to understand how to best utilize replication.

Best way to learn it, is to look how it has been done in a working game. Fortunately, Epic Games provides a free example of such game. Shooter Game has all the basics of multiplayer shooting game – just the core of the game, so it is easy to understand how different components work together. You can find this full project through Epic Launcher. Go to Unreal Engine section, open Learn tab and scroll down to find Games.

Very powerful feature in Unreal Engine is the Replication Graph, and it is really useful if you are going for MMO or Battle Royale scale of games. You can find an overview about the use of Replication Graph plugin in this

UE tech-blog post

As said, this Replication Graph plugin is Early Access at the time of writing. Actually, the Shooter Game project (look above) uses a Replication Graph (similar to Fortnite), so you can learn how to implement it.