Hi Adithya,
It looks like nobody has answered your questions.
Yes, GitHub is used by many game developers for their Unity projects.
That depends on GitHub. For further information, please check their website.
GitHub is just a hosting service for git. So is GitLab. Plastic SCM is not the same as git. For further information, please refer to Wikipedia.
That depends on how well you and your team know git, and whether you decided on a workflow. If everybody in your team follows different rules, you might end up in chaos. Chaos in git is difficult to resolve.
That could be a solution.
If you need more help, please feel free to ask our helpful community of students for advice over on our Discord chat server. Stackoverflow, reddit, and the official Unity forum are also good places to ask questions.