Inventory and assembly items have three accounts
expense = COGS
income = your sales income account
asset = inventory asset
any video or advice you received showing an asset account selected in the income account selection is wrong.
The inventory help files and video tutorials are helpful. QuickBooks for Dummies, and Business Accounting for Dummies are great books. There is also a lot of information on my blog, and several "How 2" articles on the basics. QB is an accounting program and has a steep learning curve regardless of what the PR says.
In preferences>Items & Inventory>Company tab check mark that inventory & purchase orders are active, when you create the first inventory item, QB will create the inventory asset and COGS accounts, and then go for it in a test file first.
Inventory if mis-entered or mismanaged can be the worst thing to try to correct that there is in QB.