Russia has destroyed most of Ukraine's traditional defense manufacturing sector a long time ago. The problem is that Ukraine has decentralized and/or offshored a lot of its manufacturing.

There's no need for a massive assembly plant to produce a lot of drones from dual-purpose components. Most of these components arrive across the EU border and go to random warehouses that have been converted into assembly shops. There the drones are put together, flashed with the latest firmware and sent to the armed forces, where they are armed and launched.

The bigger problem Russia faces is the surprisingly sorry state of its AA. It's been designed to detect and intercept strategic bombers and multirole fighters, but it's been almost 40 years since Mathias Rust, and it still can't handle a cheap and slow flying target, relying only on point defense systems that can be and are overwhelmed. Ukraine has inherited the same Soviet tech but managed to build a better detection system that collects and processes the data from thousands of cheap listening stations across the country.