2 different airframes.

The full payload like the FP-1 made out of more durable materials and designed travel long distances and blow shit up

The "decoy" is made as cheaply as possible with a radar signature designed to look the same as the full-payload. They can travel long distances as well.

By daylight they look different, at night on radar they look very similar. A good radar technician might be able to spot the difference and not waste the ammo on a decoy.

So Ukraine puts explosives on the decoys and turns them into AA hunters. This forces the AA to fire on all contacts, expending their ammo rapidly. It also gives Ukraine a swarm of armed "decoys" to target any AA. Oh what's better, announcing that they are doing it makes it MORE effective. The only thing that AA crews can do to avoid becoming a target is to NOT fire.