Can you share some basic knowledge on Git?
Git is a VCS (version control system) that allows distributed collaboration on text-based files, usually on open source software codebases.

It was initially created by Linus Torvalds in 2005 in order to help develop the Linux kernel, which is one of the largest codebases that make use of Git, however today Git is used to maintain codebases of all manners of scale and complexity.

Nowadays, Git is most often used in tandem with a service that remotely hosts your repository, such as Gitlab or Github, but using such a service is not required nor intended when using Git, as users and developers can simply make patches to codebases and manually send them to one another over any medium, even via E-mail - the aforementioned Linux kernel codebase, for example, is still collaborated on over a mailing list, where maintainers send patches to one another.
What's the difference between gitlab and github? Which one is better? Why there's no one git service?
GitHub and GitLab are both websites that host users' Git Repositories. GitLab is more open and allows users to pay to self-manage if they want, GitHub has more features and more users. It is not possible to say which one is better as it is a subjective choice. There are many reasons why there is not just one Git Repository hosting site one of them is competition. Competition forces the people creating the product to try and make their product better which benefits the users.