Java Edition "pick block" works like so. Each bullet point is attempted, and subsequent bullet points are only attempted if prior ones failed (i.e. don't do behavior #3 if #2 already succeeded)

• If the picked item is already in the hotbar, switch to it.
• If the picked item is in the inventory, move it to the first empty slot on the hotbar after the selected slot and switch to it. If the hotbar is full, swap the held item with the picked item.
• If the player is in creative mode, create the picked item, add it to the first empty slot on the hotbar after the selected slot, and switch to it. If the hotbar is full, move the held item to the inventory.

Bedrock Edition "pick block" simply overwrites the held item with the picked item, and doesn't work in survival (this is covered in MCPE-12456).

EDIT: As of 1.2.10, point #1 has been implemented, but only in creative mode.