For a large manufacturer, it can be flashed onto the device automatically by a machine as part of the production line. That's not easy to set up, but basically something they already need to have set up (you don't want to have humans have to plug in boards to flash firmware and load serial numbers, etc. on to every unit).
Ah, I see. I thought OP used TP-Link for their router. I missed that Tapo (the camera manufacturer) is a subsidiary of TP-Link.
I think he has it backwards: Easy for a small vendor, very hard for a large one.
For a large manufacturer, it can be flashed onto the device automatically by a machine as part of the production line. That's not easy to set up, but basically something they already need to have set up (you don't want to have humans have to plug in boards to flash firmware and load serial numbers, etc. on to every unit).