I watched much where youtubers in creative mode only one stack had. But then they swiped it over the slots in the chest, and the whole chest was filled with stacks! How can they do that?
In creative mode, dragging an item (or stack) while holding middle mouse button fills the slots with full stacks of the item.
Both in creative, and in survival, dragging while holding left mouse button splits the held stack roughly evenly between slots it's dragged over and what's still held.
Similarly, dragging with right mouse button leaves single items from the stack in the slots.
Additionally, middle-clicking on a non-full stack (or single item) in creative, picks a full stack of copies of that item (leaving the original in place). Right-clicking (both creative and survival) picks half of the stack.
Additionally, shift+double-clicking on a stack in inventory with "empty" cursor while having some other inventory (a chest etc) open, transfers the whole stack to the target inventory. If instead of "empty hand" you have picked (any, unrelated) item (using left click), shift+doubleclick on an item or stack transfers all stacks/instances of that clicked item to the other inventory.
ps. middle-click outside the inventory, while having a placed block under the crosshair; "pick block" is one of the most useful functions both in creative and survival. Ctrl+middle click in creative will pick it including NBT data (chest with contents etc.)