The most useful classes will be the Connector and the Bridge utilities (Bukkit, Bungee, Velocity). Both can be accessed via the getters in the plugin's main class.
Check out the Bukkit, Bungee and Velocity bridge utility source for example implementations of the messaging.
See the GlobalWarps plugin for an example on how to use (parts of) the bridge.
<repository> <id>minebench-repo</id> <url>https://repo.minebench.de/</url> </repository>
<dependency> <groupId>de.themoep.connectorplugin</groupId> <artifactId>[bukkit|bungee|velocity]</artifactId> <version>1.5-1SNAPSHOT</version> <scope>provided</scope> </dependency>