So it is best to see Free Pascal & Lazarus as the free and open source alternative to Object Pascal & Delphi respectively.
Of course, now the term Object Pascal is not used at all. Delphi is used to refer to both the language and the IDE now.
About Free Pascal, it is largely compatible with Delphi/Object Pascal but there are minor differences. https://wiki.freepascal.org/Lazarus_For_Delphi_Users
Here is a write up on the advantages of Free Pascal: https://www.freepascal.org/advantage.var