I expect that those areas have AfD in leading positions in local governments

Then you don’t know what you are talking about.

No, it’s more like most of those regions have low economic opportunities to begin with and that’s why not many people migrate there. Conversely especially young and educated people leave those places for greener pastures.

Also note that it also makes sense that areas with higher immigration backgrounds to yield less votes (as a relative proportion of total votes) for the AFD. Simply because people with immigration background would be stupid to vote against their own interests.