Yup. Both sides of the cold war developed “permissive action links” that allowed forward deployment of nuclear warheads on foreign soil (and to prevent rogue generals who could leverage nuclear weapons for political power domestically). These are what “nuclear codes” refer to, and famously were set to 0000 0000 for the first few years in the US due to fears the codes couldn’t be dispersed in time of nuclear war.

Ukraine’s warheads were essentially inert, but any detected attempt to break the C&C would’ve probably prompted an invasion of a scale much larger than the one we’re witnessing now, so it made sense for them to transfer the nukes back to Russia.