What discords or resources should i know or get started with when getting into java plugins

Too many developers go missing or AWAL for no reason when working on plugins, id rather just learn about this myself, I have a JAVA plugin i paid a guy to work on, he hasnt gave me the code via github, is there a way to look into a jar and review my code for research / expanding my plugin?

Also what are the discords or websites i should know for making java plugins.