The way that AWD vehicles work are all over the internet...and you'd probably get a better & well written explanation versus us trying to fill in the blanks...not knowing what you already know or don't know.
To explain how BMWs original AWD X5s work would be redundant when the info is already out there. And with much respect...it would take a lot of info to get you up to speed if your perception is that turning the wheels on the rear axle (behind wheels) should also turn the front axle (ahead wheels?) ... since the X5 of your era uses a center differential (not locked) which uses (I believe) a 60/40 power split as a default driveline.
Here's an example of the type of information out on the internet that may help you understand better:
All Wheel Drive Explained | awd cars, 4x4 vehicles, 4wd trucks, 4motion, quattro, xDrive, SH-AWD, Haldex, Torsen, wiki - How it works
Read the article at the link above to educate yourself on the various AWD systems...then if you have more questions...feel free to ask for more clarification.