The population benefits in that, all other things being equal, the larger the collective, the stronger the effect of the division of labor, and as a result, the higher the standard of living.

But this is all in theory, and in practice, quantity does not always mean quality. A prime example: Russia (who would have thought?!), with a population of over 100 million, has a standard of living that leaves much to be desired, not to mention the rapid decline in that very population. Or India - the second most populous nation on the planet, if I'm not mistaken, and its people live like savages. But in Austria, with a population smaller than that of Kyiv, there is some kind of unhealthy order. I'm not even talking about places like Switzerland, etc.

I'm interested in how the lives of the people of Ukraine and Russia will specifically improve (if I understand you correctly, and you meant ordinary people) after unification? At what cost? Is there a development plan?

Russia's colonial policy, both under the tsars and under the general secretaries, was the opposite - the periphery was lifted up both economically and infrastructurally, protected from external aggression, and even the local elite was not particularly humiliated, but integrated into the existing power hierarchy. Even in Afghanistan, during the war, they built so many infrastructure facilities that the Afghans still remember how well they lived under Soviet "occupation." And they compare it with the Americans, who only shoot and support drug production.

First of all, I wouldn't compare monarchy and communism with modern democracy in Russia. The interests are different now.
Secondly, things are far from perfect for you, both economically and infrastructurally. And you want to lift up other countries? Maybe you should start by putting your own house in order?
I don't really believe that Russia is so eager to share its invaluable experience and lift Ukraine to new heights. Rather, it seems that Ukraine is needed by Russia for the same reason that it is needed by the United States - as a source of raw materials. Your article even states that industry is needed. 46 million new slaves is also a considerable asset. And, of course, to bring the local capital under control - that's a must! Oh, and to push NATO missiles further away and place its own, how could it be without that...
So, no matter how you look at it, you still have a lot of work to do (first of all, on yourselves) to convince us that it will be better for us with you. So far, there is no such trend... unfortunately