How to know a community of researchers in order to be more successful in research and getting papers accepted?
1) Communities usually revolve around research topics, rather than journals. There are smaller and broader communities. The smallest ones are about a very specialized topic that only three research groups in the world are knowledgable about. The broadest ones are about an entire overarching research field. Of course, there also are overlaps between communities.

2+3) In my field, the only correct answer would be: Go to meetings and conferences. This is the place where people go to exchange ideas, in talks and informal conversations.

4) This is a very broad question. There can be all kinds of relations between communities, personal ones (overlaps) and emotional ones (respect, envy, disregard).