There is no default warehouse in Store Commander.
We made it simpler: ticked boxes located in the 'Warehouses' grid in the Properties panel.
It allows to associate one or multiple products to one or more warehouses in bulk and in a second.
Also use the mass-association/dissociation icons on the toolbar once you have selected the concerned warehouses beforehand (the boxes will then be ticked/unticked for you), as shown on the below screenshot (Warehouse grid is located in the Combinations panel):
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