Resources for explanation of GFP usage in C elegans for gene expression, etc

I am an undergrad freshman, I believe I understand all tools to study c elegans except this GFP analysis for gene expression. No book seems to talk much about it
I do understand the core concept that this is a reporter gene but how its introduced and how it spreads to the whole body of the c elegans including each cell is confusing me.
Please help with any resources