This is the real motive why the war started,the zones in which Russia claim to have been had referendums wanting to go to Russia had only sham referendums and the reality is that they are rich in lithium,yes the substance that makes the batteries for electrical cars,it is a huge market this one and Russia does not want to miss out,they have their ill imperial fantasies since centuries now.The second is that through these parts they can cut Ukrainian grains from shipping from such ports and ship their own grain,in the end every war has invading fantasies that will make the country that does this war richer,it is the same always.
In accordance to the constitution of Ukraine - there's no need to hold any referendums:
"The land of Ukraine and minerals in it belongs to the people of Ukraine. Property rights on behalf of the people of Ukraine are delegated to the heads of regions (oblasts)" (c) Constitution of Ukraine.
So, basically - a head of oblast can do any sh*t he wants, like join USA, and the only need for referendum is to convince USA that the people, indeed, want to join it. Or else - there are no property rights and the Constitution is just an expensive substitute for toilet paper.
There is definitely a certain percentage of "pro-russian" population in Ukraine - and either it'll be eliminated, physically, or they have the right to join whomever they want - taking what belongs to them according to the laws, that is. I don't know what's for do we need that kind of people and each elections splitting the country by half, or parties activities, church being forbidden and so on. It is impossible to build any democracy if there are existential incompatibilities inside the country unless politicians will stop the radicalization of the society inside it or those incompatibilities get eliminated one way or another.