p/join is quite a low level function at this stage, its still ok to use to use though, the point of join is moving the parsing context to a new one, so that usually happens when you have a join in your query, the arguments are the new entity data and the environment, the children AST will be extracted and a new instance of the parser will be called to process the sub-query

there are many details that evolved along this process, I suggest you try reading the sources and see if that makes sense, if you have any specific questions around it please let me know