Area2D under the Shared Areas node in Spawning . They determine the physical layers / masks of the bullets being spawned.

Basically, every bullet is a collision shape belonging to this area.

If you want to spawn a bullet with a different layer / mask, create a new Shared Area with those layers and indicate you want to spawn the bullets with its physical properties by changing the shared area property of a SpawnPoint or by calling the Spawning.spawn() function with the correct parameters.