What is the actual implementation of delphi helpers?

I think this is best explained with some code:

uses
  System.SysUtils,
  System.Rtti;

type
  TFoo = class
    procedure X;
  end;

  TFooHelper = class helper for TFoo
    procedure Y;
  end;

procedure TFoo.X;
begin
  Writeln('TFoo.X');
end;

procedure TFooHelper.Y;
begin
  Writeln('TFooHelper.Y');
end;

var
  f: TFoo;
  ctx: TRttiContext;
  t: TRttiType;
  m: TRttiMethod;
  i: TRttiInterfaceType;
begin
  f := TFoo.Create;
  f.X;
  f.Y;
  // Looking at the assembly code for these two calls will show this 
  // (remember to turn off ASLR if you have a recent Delphi version as that
  // disables the possibility to see the method names):
  //   call TFoo.X
  //   call TFooHelper.Y

  Writeln('Methods of TFoo:');
  t := ctx.GetType(TFoo);
  for m in t.GetMethods do
    Writeln('  ', m.ToString);
  // Okay, we don't see that method there, what if...

  Writeln;
  Writeln('Methods of TFooHelper:');
  t := ctx.GetType(TypeInfo(TFooHelper));
  for m in t.GetMethods do
    Writeln('  ', m.ToString);
  // Ah! And it looks like it was inherited from TObject, or? Let's see...

  Writeln;
  while True do
  begin
    t := t.BaseType;
    if t = nil then Break;
    Writeln(t.ToString);
  end;
  // Oh, there is that TClassHelperBase, and it inherits from TInterfacedObject - I wonder if...

  Writeln;
  t := ctx.GetType(TypeInfo(TFooHelper));
  if t is TRttiInstanceType then
  for i in TRttiInstanceType(t).GetDeclaredImplementedInterfaces do
    Writeln(i.ToString);
  // No, it does not implement any other interfaces.

  Readln;
end.

Things get a bit more interesting when you implement virtual methods on the helper because then the compiler puts some interface onto that generated class which you cannot easily get via the GetDeclaredImplementedInterfaces because their TypeInfo is nil. But stepping through the call of a virtual method call will show how it works internally.