An infantryman in the end needs infantry training. Key parts of modern infantry training were established no later than by the end of WW1. There is no magic here, just the available capacity of this training(the more instructors - the more personal time and attention an individual conscript will get - the better are his chances to not die due to his or his squad leaders' basic neglect and mistakes).
Given how Ukrainian defense (and actually offense, too) works* - they don't really need supersoldiers. They need a lot of them with at least basic skills(and as we can see, even really badly trained ones are actually resilient enough when backed up by those who know what they're doing), though, and mobilization provides Ukraine with ample supply of that resource.
Russian army, on the other hand, had huge problems with anything infantry since the 1960s (there is almost no true infantry in RU army in the first place), and it really shows - there is simply no men to even properly control that it has, much less to attack more.
*basically infantry screen (of company strongpoints) held together through artillery fires. It's really cynical of them, but if reinforcements come up faster than Russian artillery grinds them out - and infantry in camouflaged field fortifications is quite resilient to fires, - the ratio works out for the Ukrainian side.
The cringeness of it aside are the Russian units in the south that originally launched from Crimea still supplied from Crimea? Because that's actually a really long supply tail if the supplies for them actually come from Krasnodar or something.
Crimea itself is a mighty logistical hub.
