In years gone by, there was a “Default Warehouse” field in the Item record and you would set it as you created any new Items in the database. Like this:
“Sales, Purchase, Accounting Defaults” Section. They did this so that you can set default warehouse by Company in case you wanted to share a particular item between multiple companies. If you don’t see the field, just click on edit for that row that has the company you want, and you can set it there.
I would not have found that because the section is blank with no rows when you first create a record. You actually have to click “add a row” for your company name to show up in a new row and then you can alter the warehouse.
To add, you can set the default warehouse for ALL items (unless you over-ride at the item level) in the “Stock Settings”. This warehouse will be used for newly created items that do not have a default warehouse specified for it.