Basically Aktiengesellschaft (AG) or Group refers to the same entity. Some people use AG meaning just VW brand. But it's not totally correct.
The brands highlighted by the other user basically include all the vehicles managed by VW group
My suggestion, if you need correct figures, is to use the ones delivered in the annual report (you can simply download it from the corporate website, volkswagenag.com). Those figures are reliable anyway
Just one note: You wrote that Fiat Group (actually Fiat S.p.A. after the demerge) delivers separate figures for cars and commercial vehicles. Beware: commercial vehicles are, for the most part, branded as Fiat Professional, which is included inside Fiat Group Automobiles (together with Fiat, Lancia, Alfa Romeo, Jeep and Abarth, plus Chrysler branded cars for UK and Ireland).
There are some commercial vehicles not managed by FGA, but they are managed by Iveco (LCVs, middle size trucks and heavy trucks). Iveco is not part of Fiat S.p.A., it is under Fiat Industrial S.p.A.
These two companies are now totally separated under a legal point of view, and have different reports, stocks, etc.
On the other hand, what you could/should consider under Fiat S.p.A. is Chrysler Group LLC, (anyway, if you check the annual report the financial figures are already consolidated)