How to communicate between Client and Server?

Newcomer to AWS and GameLift, so this might be a really stupid question.

I have set up my server and client correctly, as far as I know, and they interact correctly with regards to game and player sessions when I try testing them with the local GameLift as per this page. However, I can’t find any methods for sending/receiving messages. Are there any? Are they in GameLift or in another part of the AWS API? Do I code that part myself, and if so, how do I use the sessions?

Sorry that this seems such a basic question, but I can’t find any info about it. I’m developing the game in Unity, if that’s any help.

UNET, low level api, or high, sessions only for connecting (on client) and accepting (on server)