This sounds like a textbook question. In fact, I am pretty sure I answered something like this last year for the MEP (Moral Education) exam but exam answers don't always reflect reality since I cannot write a 1000-word essay in the answer sheet! Anyways, I think we should first start by defining what is a community. According to Oxford Languages, a community is "a group of people living in the same place or having a particular characteristic in common". I think being in a community is not the same thing as 'feeling a sense of community' and I'll tell you why.
Let's look at the Americans as an example. They are a community, as they live in the same place and they have some characteristics in common too: we can say that most of them speak English, for example. However, despite them being in one community, some Americans can't stand each other. Whether it is due to their political leaning or religious beliefs or race, whatever. Essentially, the sense of being in a community is not there among all of them. I am not saying that's a good thing or a bad thing, it's just something that's present and if I'm being honest it's not only the Americans: even in my country India we see this.
This whole thing actually reminds me of a scene from a movie. It was towards the end of the movie so if I write down the name of the character who said it and the movie, it will likely be a major spoiler and I wouldn't want that. So basically this guy wanted a war between US and Russia and he was trying to incite Russia so it launches an attack on the US. Why would anyone want that? When our main character asks that to the guy, he replies that the Americans needed a common enemy to remain united. Otherwise, we will just fight among ourselves (which they were at the moment): that sense of unity is important for everyone in there to feel a sense of community.
So now the question is, "where do I feel that strong sense of community" and not just "which all communities am I a part of" unlike the exam paper. So one of the first communities I can think of at the top of my head is my classroom. If I look at it, it's like one of the best communities I am in: I know everyone there, we all get along very well, we have each others' backs and we work together towards (mostly) common goals.
Another one is my friend group but I don't know what I can name that community. You see, throughout the pandemic I didn't really talk to the people in my class properly. After I met them post-pandemic, I realized I had gone almost an year without personally speaking to them. Now this happened because our school had disabled all chat features from Teams so we couldn't even chat with anyone (teachers included). Ironic given how Teams is a communication platform foremost but anyways. When this happened, people just fled to social media platforms to talk to each other because, let's be honest, if our school expected that for ~2 years we won't talk to each other then that is very, very naive of them. The most popular platform for this was Instagram and in 2020 I had a good presence over there. Now close to the end of 2020 (I think), I reduced my Instagram usage and come 2021, I was almost off it. Throughout 2021 I had points where I came back and then left again but as of now, I've not touched Instagram at all for the past 3 months or so.
Anyways, coming back to the point: I had to make friends online now because that's how the world was in 2020. So I made those friends on Instagram during 2020 and then in 2021 most of them moved to another platform. There are still some of them who have Instagram but I don't talk to them there. There are also new people who continue to join on Instagram but I don't really have contact with them either, at least not on Instagram. So I ended up having this group of really close friends, both from my school and outside my school. The very first of those friends were some of my classmates and then later it expanded to people from outside my class and in due time, people outside my school too. I am very satisfied with how things are now and I love the people I talk to. I don't want to change unnecessarily now.
Whenever someone asks me for my username in offline life, I give it to them but warn them that I will likely not respond anyway. After 2 years of being online, I want to enjoy and cherish these offline memories! And as I said earlier, I don't want to add more people online that I already know and talk to offline.
The new (online) communities I joined this year are DALL-E, because I am a part of OpenAI's DALL-E 2 beta, and Midjourney, which was invite-only when I joined them but now they are open to all. I would love to have these offline as well but I obviously cannot because there are people there from all over the world. I mean, that is what online communities are for, isn't it?
Now basically the thing here is: I don't want to be a part of any new communities online when I already am a part of them offline. Being constantly reachable by tons of people is a headache I don't want to deal with! It's tiring and it just hurts the brain, it messes with your mental headspace.
So, well, to answer the question in the prompt: my classroom (offline) and my friend circle (both offline and online) are where I feel that much-needed sense of community.