> It's not that Russia had nukes in Ukraine and withdrew them.
Russia is the single legal successor of the USSR, so all Soviet nukes became Russian nukes, regardless where they were located. So after the USSR broke up, Russia did have nukes in Ukraine and withdrew them.
Legal succession is mostly irrelevant and more complicated than that. Russia had operational control because it had taken physical control of the ex-Soviet command and control systems which were in Russia, and hence had the launch codes, etc.
To be fair, Russia becoming the single successor of the USSR wasn't a foregone conclusion in the early 1990s. There wasn't relevant precedent of a country dissolving I think -- Yugoslavia was still battling it out, Austria-Hungary was too long ago.
It was an explicit decision by both CIS and UN. Russia took USSR's seat on UNSC two weeks after USSR was dissolved, and that happened in 1991. Budapest Memorandum was negotiated 3 years later, by which time this was already a firmly established thing.
Russia is the single legal successor of the USSR, so all Soviet nukes became Russian nukes, regardless where they were located. So after the USSR broke up, Russia did have nukes in Ukraine and withdrew them.