the webpage xr7kid posted is very helpful! All the information I need to know where the hidden clips are when removing panels and trims!
regarding BMW genuine roof rails, I asked two different local BMW dealers and both told me roof rails is "manufacturer option" meaning need to order when you purchase the car, cannot be installed at later time (at least for Japan this is the case)
Hence the best option for roof rack system on normal roof, you can get 3rd party kit from company like Thule:
https://www.thule.com/en-us/roof-rac...dr-suv-_-ZLuM#
The mounting kit 5165 is for X2, and the rest of the components are universal. You can find youtube videos on how these mounting kits that clamps to the roof and door-frame works.