Well, we didn't conjure a whole new army out of thin air. Russia still has more meat for the grinder than we do. We have better tech, though, which helps to equalize the advantage while also pressuring russia economically and logistically.

That is the oversimplification of the situation. Ukraine has stalled Russian progress in almost all locations. Ukraine is severely disrupting Russian logistics, there is a lot of reports about it, but none of them are explaining that doesn't mean stopping Russians. So Ukraine does have a lot of success. Russia can still progress further with it's tactic of infiltration of small groups. And it is true that the tactic causes incredible amounts of casualties and it's true that Russia currently can't replenish the loses, however they can find ways to conscript many more people so the current issues are not worrying for them.

In any case, they can't get Donetsk without a serious mobilization of hundreds of thousands of people. But they can do that.

I think the question is how. Full scale mobilization or some weird scheme they figure out when they don't have to do the official mobilization.

Infiltration teams can capture empty land. A big city? In whole of the war, Russia has not really taken a city this big. Even smaller cities, they needed a year for and absurd amount of casualties. They only took big cities and the beginning when they were not defended. This is going to be a bloodbath for Russia. Ukraine is prepared and the ratio of casualties will insane.