Do you want to detect the item by ID, name, or tag? This applies to all cases. Let's start by detecting it by ID only:
execute if entity @e[type=item,nbt={Item:{id:"minecraft:diamond"}}] run
The command above detects any diamond item on the ground and works with any entity. After run, you can choose the command to execute. The same logic applies to the next commands.
Now let’s detect it by tag:
execute if entity @e[type=item,nbt={Item:{components:{"minecraft:custom_data":{Teste:1b}}}}] run
By name:
execute if entity @e[type=item,nbt={Item:{components:{"minecraft:custom_name":"Testando"}}}] run
And we can also combine all of them like this:
execute if entity @e[type=item,nbt={Item:{components:{"minecraft:custom_data":{Teste:1b},"minecraft:custom_name":"Testando"},id:"minecraft:diamond"}}] run
This way, we can detect a diamond item in a very specific way.