Detect player’s items

This is a "theoretical": it isn't a creation, but it could be used as a part of some creations.
In this page I left only the "mechanics".

Difficulty: EASY (2/5)
Commands in the mechanic: NORMAL (6)
External commands: LOW (1)

/scoreboard objectives add item dummy

Adds a score called "item", that can only be changed with commands

/scoreboard players set @a item 0

Sets at 0 the score of all players: this is the base of the "auto-resetting" mechanism

/scoreboard players set @a item n {Inventory:[{id:"minecraft:id"}]}

Sets the at "n" the score of all the players with an item ("id") in the inventory

/scoreboard players set @a item n {Inventory:[{Slot:xb,id:"minecraft:id"}]}

Sets the at "n" the score of all the players with an item ("id") in their "x" slot

/scoreboard players set @a item n {SelectedItem:{id:"minecraft:id"}}

Sets the at "n" the score of all the players that are selecting an item ("id")