What to do in Milestone 2?

What to do in milestone 2? What newly created implementation refers to? Is this different from PortfolioManagerImpl.java?

It refers to PortfolioManagerImpl itself.
You dont have to create any new java file.
I hope it helps

Refer to the references given in their sections, regarding interfaces, how it works, look at the code samples given, you will find out what newly created implementation refers to

We just have to make the calculateAnnualizedReturn() method defined inside PortfolioManagerImpl work