SQL Server was Sybase until (I think) version 4.9, just rebranded as Microsoft SQL Server.

Then the two versions split and I don't think that any of the Sybase source code remains in what is SQL Server today.

That said, a lot of the concepts (like a significant number of system stored procedures) and also TSQL remain almost the same, with small differences (except for system functions, which SQL Server has a lot more functionality).

When you come from the Sybase world getting a start on SQL Server is quite straight forward when it comes to handling the database.

Internals and other low level nuts and bolts differ nowadays, of course.