In this video, he takes a single ender pearl and creates full stacks from it, what's the keyboard command to do this?
Middle-click (depress the mouse wheel) on an item in inventory/container in creative mode to pick a full stack of that item.
Dragging over inventory slots with the wheel depressed puts full stacks of the held item in the slots dragged over, like in the linked video. (dragging with RMB places single items, and with LMB - splits the stack held evenly between the slots)
Additionally, middle-clicking while looking at a block placed in the world, puts a copy of it in a hotbar slot in creative mode, and moves a stack from inventory to current hotbar slot in survival mode (if you have any).
Shift + Click. This will "select" a full stack of the item you click on.
In the survival inventory tab of the creative inventory, Shift + clicking the X clears the whole inventory. In any other tab, shift + clicking an item puts a stack onto the hotbar