The VW/Audi relationship is not a hard thing to explain. They are the same company. Audi is luxury arm of VW the same way Lexus makes luxury Toyotas and Accord makes luxury Hondas. Same company. Porsche is not the same company although there are working relationships and they often use the same parts, etc. In the old days VW/Audi and Porsche were all imported to the US by the same importer, called VW of america. There are also historical relationships between the car makers themselves, most famously that Ferdinand Porsche designed the original VW beetle and that these mechanics were integral to the Porsche 356 and later 911s.