Doesn't China produce Russian 152mm shells and artillery for export, even if the PLA itself is moving to 155mm? Plus, Russia prior to Ukraine had tons and tons of Soviet surplus in storage. It has taken over 3 years of attritional warfare for Russia to start running out, and even then they're still outshooting Ukrainian artillery with Western support, if I'm not mistaken.
Surge of Russian artillery shell production since 2022 tells you they are not just using their Soviet era stockpiles alone. There are reasons why. It can be because of the way the Soviet shells are mass produced, the manufacturing tolerances are loose and varied. If you have slight difference in your shells in terms of weight for example, or the shape of the shell which affects the aerodynamics, it affects the calculated predictability where the shell will fall. If you have a wide manufacturing variance, the shells will land all over the place, which is consistent to observation during the first year of the war. The Russians are spamming shells all over in the hope they hit something, and when a 152mm shell has a 50 meter blast and fragmentation radius, it will frequently apt to. Even a 122mm shell still has a nasty 30 meter AOE radius. But as the Ukrainians dug in with more elaborate trenches, bunkers and dugouts, spamming has become less and less effective.
Switch to today, and it's the opposite. The Russian artillery approach has moved from spam to precision, Newly manufactured shells built to tight tolerances, targets are hit more accurately and with more consistency, sometimes in the first shot. New shells might also have aerodynamic refinements in the shape of the shell, leading to longer, undocumented ranges. Guided shells like Krasnopol are frequently used against enemy artillery, bunkers, dug outs, trenches and positions of enemy UAV and drone operators. Using a UAV to laser light a spot, these shells can land exactly right into the hole of a dugout, the door of a bunker, into a mortar site or artillery pit, or they can chase a moving target.
To survive, you need to fire as few shots as possible. The sooner you get the job done, the sooner your SPG can scoot out of there or the sooner you can hide your field howitzer under the camouflage. Enemy UAV search for gun flashes and smoke to identify locations of artillery. Counterbattery radar will detect then track your shells back to the origin spot.
Previous post is a good example of what happens if your artillery unit gets discovered. The Orion UAV uses a thermal camera which can be used to spot a gun firing at a distance.