What's the difference between Russia joining NATO and Ukraine joining NATO?

The purpose of joining them is different. Very simply, Ukraine's accession to NATO can be regarded as the normal expansion of NATO, and if Russia joins NATO, it will lead to the disintegration of NATO. Because at the beginning of its establishment, Russia (Soviet Union) was NATO's biggest and even only imaginary enemy. Even though its functions were adjusted later, the setting that Russia is regarded as the biggest imaginary enemy has never changed.