BGP best-path selection is what determines what BGP path ultimately gets into the routing table. The AD is not used as part of this process. The AD used is ultimately a result of the best path algorithm. So, if after going through the best path selection process the route selected happened to come from iBGP the AD will be 200 and if from eBGP it will be 20 but those numbers do not factor into the initial decision.
To understand this you have to read the BGP best selection algorithm
The above PDF file will definetely tell you how the best path is selected in BGP