There's no single source, it's basically all the war reporting. My claims are not contentious. Even Russia's war bloggers are repeating the same now.

Russia could, in theory, use it's greater number of people towards producing drones. But it hasn't. Russia could, in theory, train its new recruits properly before throwing them into hopeless situations. But it hasn't. Russia could, in theory, operate by rewarding production contracts to the most capable teams rather than the ones with the best connections. But it hasn't. And even if Russia does, they'll have to catch up. They could!

Even the US could, in theory, start learning from Ukraine or even following in its footsteps, independently, but it hasn't.

Ukraine is fighting for its life, it's on Death Ground, in the terms of Sun Tzu. In Russia, perhaps only Putin is on Death Ground, and even then, there's many ways Putin could give up on the war and still stay in power. That produces far different results in people. And the cultures of Ukraine and Russia are fundamentally incompatible, which also produces very different results from people.