long before

Microsoft started the Windows transition to git sometime before Github acquisition as a part of a company-wide effort to move to VSTS/Azure DevOps Repos internal dogfooding. The choice there was between the zombie ghost of TFS source control or git, so you can possibly imagine some of the reasons why they chose to put a lot of time/effort into making git work for themselves. The subsequent acquisition of Github seems to have made them even happier at the switch to git.

As Scalar README says, is it merged to microsoft/git ?

Microsoft is trying to get away from needing a VFS. Scalar eventually accomplished that by combining some enhancements that had been added to git since they originally designed the Git VFS, and plus some new features that only live in scalar.

The long term plan is for Scalar to either go away completely, or possibly just be a simple front end for setting up a repository to use certain optional git features.

They actually have a version of scalar in official git's contrib folder, and they are very actively working with the core git team to convert the relevant features into core git features in whatever way the core git team is happy with.

But of course the present situation is certainly confusing, and does not seem to be well documented. I've only picked up on some of this from reading the git mailing lists, and have no idea how to actually use scalar.