JuhaManninen, I think the converter was using the Delphi unit but, as I can't now run it, I can't reproduce it to check.
No, the converter does not use anything from Delphi. Usually Delphi is not even installed in the machine where Lazarus runs. For example I use Manjaro Linux and Delphi would not work here. It would not work in MacOS either.
Note, the converter lets you choose between Windows only support or a cross-platform support.
If a program / application depends heavily on WinAPI, making it cross-platform requires effort. However a part of WinAPI is supported for cross-platform applications by LCLIntf etc. units.
In any case the automatic conversion usually is not enough. You must do manual changes, sometimes a little, sometimes more.
If an application uses some big Delphi component which is not ported to Lazarus, the conversion becomes very difficult.