The same place it did during soviet times, he is going to cut back on everything else and funnel it all into the “Defense” sector.
The Soviets at the peak were literally spending a 1/4 of their GDP on defense. There are going to be some unforgiving winters ahead for a lot of Russians.
I wonder if Putin sees the price of oil going back up in the near future? That would seem to be the only way they could afford this type of buildup; the old Soviet model of spending a quarter of your GDP on the military doesn’t work, no one can keep up that level of spending during peacetime.