Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

What do you call someone who knows all the gossip and stories about people in his neighborhood or surroundings, someone who's always informed. It doesn't have to be expressed in a word. Could be a sentence (e.g. she knows the ins and outs), but I'm looking for a better way to express it.
You could descirbe them as being in the know. While this often refers to someone knowing confidential information, it can also extend to someone who is just generally well informed. Collins Dictionary If you are in the know about something, especially something that is not known about or understood by many people, you have information about it.

How does one correctly apply 'in which', 'of which', 'at which', 'to which', etc.? I'm confused with which one to apply when constructing sentences around these.
The trick to knowing how to use; of which, at which, in which, to which, from which is to analyse the prepositional phrases, phrasal verbs, verbs and prepositions: He /spoke of/ war and peace and many other topics that day. The topic /of which he spoke/ was complex. The verb here that means to speak about a topic is /to speak/ of a topic/: to mention The party /at which/ he spoke/ was noisy. Phrase: A party is held /at a place/. It is implied. The situation /in which/ we found ourselves was dire. Phrase: /to find oneself /in a situation. The bonds /from which/ we broke free were tight. phrase: to /break free/ from bonds. The town /to which/we were driving was 50 ks away. phrase: to /drive/ to a place. Summary (and not a complete answer but a general one): The preposition depends on the verb that takes a preposition, a phrasal verb that includes a preposition, or it depends on the prepositional phrase used. Also, there are many other prepositions that can be paired with which: under, during, about, over, etc.

What do you call someone who knows all the gossip and stories about people in his neighborhood or surroundings, someone who's always informed. It doesn't have to be expressed in a word. Could be a sentence (e.g. she knows the ins and outs), but I'm looking for a better way to express it.

You could descirbe them as being in the know. While this often refers to someone knowing confidential information, it can also extend to someone who is just generally well informed. Collins Dictionary If you are in the know about something, especially something that is not known about or understood by many people, you have information about it.