On the Java Edition of Minecraft, when the scroll wheel/middle click is initially pressed, the thing you were looking at previously is in your hand. On Bedrock Edition, the item does not appear in your hand until the middle click is released.

To Reproduce

Load a world in Creative Mode. Look at a block/mob and press the scroll wheel like a button. The block/mob you were looking at doesn't appear in your hand until you release the scroll wheel button.