How AWD works in 330xi or 330dx?

[Oxy-]

Hello,

maybe someone could point to a place with an in-depth description of how 2001 330xi or 330dx AWD works? I read that they use electronics to make them AWD, but I'm quite sceptical about is it really possible to have a good AWD without decent mechanic limited slip differentials (say viscous or clutch or torsen type or ...) at least in the back and in the middle between axles. Or maybe they do have some?

Thanks for information,
Oxy-