How to look what other players inventory contains?
Solution 1:
To look a player inventory:
/data get entity @p Inventory
To look a chest inventory:
/data get block 0 0 0 Items
To set a player inventory in a chest and clean his inventory (command block):
execute at @p run data modify block 762 70 1565 Items set from entity @p Inventory
clear @p
Solution 2:
Provided you are using a multiplayer server, the common (and useful) Essentials plugin comes with an in-build command to view a player's inventory. You can use the /invsee <name> command to achieve this.
You can also modify a users inventory using this command.