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.
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.