It’s actually not in the code! There’s a separate file (extension: .yaml), which contains all the form’s layout information.
A base class for the Form is derived from this layout file, but it’s not visible anywhere. Because you can change the layout at will, in the IDE, the base class is computed immediately before use. It doesn’t stick around where you can see it.
Edit: this is in contrast to Embarcadero’s C++ Builder (and other tools), which attempt to edit your Form class directly. In those tools, adding a button does add it directly to the Form class’s code, where you can see it. (And accidentally screw it up!)