Dude there are only 1-2 cars in VW/Audis lineup that are similar to eachother
the VW Rabbit/GTI use the same chassis as the A3/TT, they share a bunch of components, and thats it. All the other cars use different chassis, engines, drivetrains etc.
you are quite incorrect. my 1999 a4 uses the b5 platform which is the same one used by the passat.