The 2014 referendum was held _after_ the Russian military annexed Crimea, and Russia sent the only allowed poll observers. The ballot had only two options (join Russia or go back to the 1992 constitution); the option to remain part of Ukraine was omitted.

The turnout was supposedly 83% and 97% approved the Russian option, which meant nearly all of the ethnic Ukrainians and Tartars would have had to approve. But the Tartars actually boycotted the election.

Typical Russian election. The overwhelming majority of the UN refused to accept the results.

Putin has openly admitted all of this.