If you are new to VAG vehicles or the systems on your cars, you need to understand the basics of how they work. These guides give you the overviews of that and are best to study first if you are new to all this!

http://www.volkspage.net/technik/ssp/index1_eng.php

http://www.volkspage.net/technik/ssp/index3_eng.php

http://www.volkspage.net/technik/ssp/index4_eng.php

Examples of some of the ecu systems evolutions you'll encounter (some translations needed depending on your languages);

Old skool;
http://www.volkspage.net/technik/ssp/ssp/SSP_4.PDF

http://www.volkspage.net/technik/ssp/ssp/SSP_44.PDF

http://www.volkspage.net/technik/ssp/ssp/SSP_69.PDF

http://www.volkspage.net/technik/ssp/ssp/SSP_87.PDF

http://www.volkspage.net/technik/ssp/ssp/SSP_94.PDF

http://www.volkspage.net/technik/ssp/ssp/SSP_95.PDF

Motronic/ME/OBD/CAN etc evolutions:
http://www.volkspage.net/technik/ssp/ssp/SSP_210.pdf

http://www.volkspage.net/technik/ssp/ssp/SSP_213_d1.PDF

http://www.volkspage.net/technik/ssp/ssp/SSP_231.pdf

http://www.volkspage.net/technik/ssp/ssp/SSP_238.pdf

ME7.1.1.
http://www.volkspage.net/technik/ssp/ssp/SSP_249.pdf

T/FSI Overviews;
http://www.volkspage.net/technik/ssp/ssp/SSP_322.pdf

http://www.volkspage.net/technik/ssp/ssp/SSP_337.pdf

And so on. There's bits in other ssp's that can be vehicle specific but can be for your vehicles too or your systems with some slight mods, for example 249 is W8 based but is ME7.1.1. so is easy to work out on your 6 cyl vehicles.

Once you have a basic understanding of the system then it's time to look at the thousands of maps that dictate how it all works through your ecu's!