I have worked a bit on things related to the ISS. From what I learnt, it is basically two space stations stuck together with an adapter. The smaller Russian part was launched first and is self-contained, basically a MIR 2. The US/international part was then added piece by piece, with each part having a specialized function.

Visiting Russian spacecraft can only dock on the Russian side and the Space Shuttle only on the US side, unless it brought an adapter.

I'm sure there were still many compatibility issues. For example the European ATV docks on the Russian side.