Minecraft Creative – Click Duplicate Items

This was a trick he did, he's simply hollowed out the area behind the portal and filled it with netherrack.

If you watch the entirety of the clip you see a creeper spawn in the area, as it is still part of the overworld.

This is possible somewhat easily since 1.13 thanks to /execute store.

As a preparation, you need some scoreboard:

/scoreboard objectives add temp dummy

Then you need the value 1 somewhere. Here I set it for the dummy player #1 (which can't be a real player name and does not appear in scoreboard displays):

/scoreboard players set #1 temp 1

When you actually want to decrease the item count, first store the amount in a scoreboard:

/execute store result score @s temp run data get block ~ ~ ~ Items[{Slot:0b}].Count

I specifically check for Slot:0b instead of simply using the index (Items[0]) as a workaround of a bug.

Now you need to subtract 1:

/scoreboard players operation @s temp -= #1 temp

And store it back to the chest's NBT.

/execute store result block ~ ~ ~ Items[{Slot:0b}].Count byte 1 run scoreboard players get #1 temp