Inventory Items

I Need This Type Inventory Items Entry

Manager already has this feature. You should use inventory kits.
read this guide https://www.manager.io/guides/9629

Not sure how this relates to inventory kits, but if you need extra fields to enter your specific Inventory data requirements then you would create “Custom Fields”, found under Setting tab.

The screenshot provided shows a secondary UOM and the number of inventory items contained.

Yes, but both relate to the same Inventory Item not a combination of Inventory Items such as with Inventory Kits.

The Inventory Item is recorded as Pieces, purchased as cartons with a Conversion Rate of n, where n equals the pieces per carton.