My understanding is the blocks were made by BMW to Alpinas specifications. Its basically a m62 block design with a bigger bore and a different crank to increase stroke.
Same engine used in the e53 4.6is, although the story goes that BMW liked the modifications Alpina made so much that they bought the rights to put them in the e53.
The heads are a different casting too - so you cant swap the much better b46 cams into the b44.
There is lots of conflicting info out there, but Alpina ported and polished the b46 heads by hand for their cars, whereas BMW didnt do this step with the b46 they put in the x5.
Alpina did lots of cool subtle things in their design too, like fewer oil rings, lighter pistons, underdriven crank pulley etc.