With Russia outgunning Ukraine’s artillery at an estimated 15-1 it looks like the main tool for halting Russia’s advance is about to start running at marginal effectiveness. Ukraine’s main industrial hubs are either already occupied, have incredibly difficult issues with supply lines, or have sustained physical damage, and artillery has been one of Ukraine’s most effective tools at slowing the Russian advance. The fact that Ukraine is simply using this ammunition faster than the west is willing to donate and faster than Ukraine’s domestic production can keep up with does not bode well for their ability to mount a meaningful defense for much longer. The last aid package passed by the US bought Ukraine only about 2 weeks worth of artillery shells.
The break up of the Soviet Union meant that lots of supply chains were disrupted and never rebuilt. For example the Russians basically mothballed their T-64 fleet because most of the factories that built them were in Ukraine. Similarly, the Russian navy has difficulty keeping their only carrier running because a lot of the shipyard infrastructure was in Ukraine.
That said, Russia was the largest post Soviet state and the most developed, so the lion's share of the arms industry was still in Russian borders. This allowed them to maintain more old surplus and develop new weapons.
Ukraine was and is much poorer than Russia (grain being somewhat less valuable than oil and gas as an export) and never really developed a fully independent domestic arms industry.