Every GameObject exists on a single layer, but Unity APIs that let you set which layers the API affect don’t directly use layers. Instead, they use layerMasks.
A layer is a standard integer, but a layerMask is an integer formatted as a bitmask where every 1 represents a layer to include and every 0 represents a layer to exclude. This means that you can pass a layer to an API that expects a layerMasks and the script will still compile because layers and layerMasks use the same underlying type. However, the API call won’t produce the behavior you expect.
For example, if you want to perform a RayCast against GameObjects on layer 9, if you pass 9 into the Physics.Raycast call as the layerMask, Unity actually performs the ray cast against GameObjects on layers 3 and 0. This is because the binary representation of 9 is 00001001 and if you interpret this as a mask, the 1s are in the place of layers 3 and 0.
For information on how to set up a layermask to use in an API call, see Set a layermask.