It's not even electrical output. They mount harness to the drone carrying payload with a 9V battery (or whatever) with a light sensitive resistor positioned over the auxiliary light. When the light turns on the payload is released with a motor/solenoid/something running from that battery on the harness and it doesn't require any modification to the drone itself.
It's and SMD led on a main board of the drone (at least on DJI ones) and the whole board is quite a complex computer with a ton of RF tech, power limitations and whatever is included to make those things both safe and fun for your average consumer. For a skilled operator it's not a problem to pull out the led and wire it to a transistor, but you need to pull the whole drone apart, somewhat sophisticated tools to solder wires to the led contact points, reassemble the whole thing excactly as it were and then connect that to the external harness.
Or, you can just bend the frame out of chicken wire, twist wires together and secure them with a tape or hot glue, zip-tie that to a drone and you're good to go. I think in Ukraine they use a ton of 3d-printed stuff which makes it more reliable and even easier to assemble. That way you don't risk breaking the drone and you can prefab pretty much the whole thing and just send them out to the field where practically anyone can assemble it even on standing in a mud puddle and have successful results within minutes from pulling a new drone out of a box.
I guess that makes sense. Easier to just assemble than disassemble+assemble. It might cost slightly more in materials and preform slightly worse, but they're primarily limited by people and time, not aid dollars to import electronics.
Pretty genius, as a drone builder myself I appreciate lots of the drones being built during this war. Just unfortunate they have to be used for this purpose.