> Given the documentation says that this is supposedly to be between JVMs, how do they handle the serialize/deserialize?

Your transfer object needs to implement MemorySerializable. Below two examples from CoralRing's GitHub:

https://github.com/coralblocks/CoralRing/blob/main/src/main/...

https://github.com/coralblocks/CoralRing/blob/main/src/main/...

The second one effectively allows you to send anything you want (as bytes) through the ring, making CoralRing message agnostic.