I am trying to look for a way in which multiple people can work on the same code collaboratively on the online Remix IDE. Currently, I have no option but to copy the updated code from Github manually and then paste it on Remix. Is there maybe some plugin or inbuilt functionality that allows collaboration? If not, what is the next best alternative?
Nope. You should use GitHub, GitLab, or some tool of that sort to keep the codebase and then manage that codebase in your IDE of choice (in your case, Remix).