I think the main problem is to see Berlin as a city. Even the official city is more a bunch of divided, but connected middle-sized cities. For example, the City Berlin has ~3.7 Million citizens, which is nearly double the size of the next biggest cities (Hamburg 1.8 Million, Munich 1.5 Million). And the region Berlin even has 4.7-6.3 Million, depending on which parts you add. Half of the German states have fewer citizens.

> 2. around ~20% of people living in Berlin aren't from Germany,

Notable part here: that's not even the highest amount in Germany. There are many cities with similar or higher numbers. But to be fair, depending on the region, there are likely many people from neighbouring countries which will have less problems with integration because of their shared cultural background. Berlin is for historical and political reasons a big melting pot of different cultures. It's not the only one, but a bit different because of it's size.