A great way to produce stuff that you want to keep secretI remember reading (sorry, can't recall where) that Ukraine has a huge cottage industry making home-made drones. I'm pretty sure it said there were thousands of households/small factories that were making them. So the Russians can't bomb the factories, as 1) there are thousands of them and 2) they don't know where they are. And it appears they can't easily defend Russian infrastructure against them either.
Here's David Axe writing for Forbes:
Ukraine Has Developed 15 Kinds Of Long-Range Strike Drone—And Has Sortied Them Against Russia’s Oil Industry
In a year of raids targeting a dozen refineries and depots in western Russia, the Ukrainian intelligence directorate has reduced Russia’s refinery capacity by 12 percent.www.forbes.com