On sheet Parts is a table, where you register parts. This table is defined as Table tParts (select any cell in table and select Design from Menu - in Table Name field in Menu Ribbon you see the name of defined Table). Here you enter all parts you have, and quantities to keep for those parts. When afterwards you get new parts to account for, you add those into tParts. When the quantity to keep changes, you edit it (as this is a very simple application, you can't keep e.g. the history of quantities to keep here). When you don't need some part anymore, you can delete according row from table, or you can set QtyToKeep = 0.

Below Table Name field in Menu ribbon is a button <Resize Table>, which opens a "Resize Table" popup window. There is a field, where you can edit data range of Table. For tParts, in my example there the data range is "=$A$1:$B$4". When you edit it to "=$A$1:$B$14", and press <OK>, 10 new empty rows are added at bottom of Table tParts.
You also can simply enter a new PartNumber into cell A5 - the Table TPards is expanded for one row.
Or you can select cell B4 (the bottom right cell of tParts, and press Tab - an new empty row is added at bottom of tParts.

The same applies for all other defined Tables (tInventories, tInventoty, and tPartOrder). Btw, I tried to remember yesterday, what more to correct, and didn't remember to correct Table name (tInvetoty > tInventory).

On sheet PartOrder, you can only select an inventory date, and resize the Table tPartOrder, when the number of rows in Table is not enough to display all parts you have to order (when there aren't any empty rows at bottom of table, then certainly resize the table). Never enter anything into tPartOrder. When you do this accidently, then select table cells from some row not corrupted, and drag the corrupted row over to restore formulas.