Okay, but I think it's a good way to learn !
https://github.com/Charlito33/Tyvalia-Mod/blob/main/src/main/java/fr/tyvaliarp/tyvaliamod/events/CommonEventHandler.java
This i the class where I wan't to sync chunk at :
// TODO : Save Chunk with Packet !
https://github.com/Charlito33/Tyvalia-Mod/tree/main/src/main/java/fr/tyvaliarp/tyvaliamod/capabilities
And this is the capabilities (IPollution), who can be attach to chunks.
(I forgot to say I am in 1.12.2)