Is there any directory on which l can get to know about open source communities on various platforms ?
Web search engine of your choice.
Keep in mind that every open source project scratches a different itch… they don’t exist because people wanted to collaborate for collaborations sake… they exist because someone (or some people) wanted a particular software capability. This means that many of them started because one person had that itch, but there are millions of itches so the projects that need your help very likely won’t fit into a convenient “top 100” list. Think about what you are interested in and search for open source software related to that topic.
INeedMana@piefed.zip 3 hours ago
FWIW, here’s part of what I’ve put into my “opensource” feed