where to set the default debit account for inventory items when creating vendor bills?

It's the Asset account on the Inventory Item record. For stand-alone vendor bills, that asset account will be debited. For vendor bills from POs, the Accrued Purchases/Inventory Received Not Billed will be debited since the asset account of the inventory is debited in the IR instead.

is there a way to change the default account for vendor bills from POs?

Item receipt

Well, i'm actually curious how it was set up in the first place since the chart of accounts is not same for all companies right? so it must've been set up somewhere. I'm not really gonna change it, just wanted to know where they set it up.

You can also setup defaults for future new Items at Setup>>Accounting Preferences >> Items/Transactions.

which field?

Please visit that Setup >> Accounting Preferences and you will see all.

the one I'm talking about is not there

@Mars to understand you want to understand how your account started using "*Inventory Received Not Billed Account".* From SA 82119 that's a Netsuite standard account and cannot currently be changed.

Thank you. That's exactly my question.