What is the difference between the Transactions/Banking tab and the Expenses tab in QB online? Specifically what updates the Expenses tab, QB or the Bank?

Let me help clear up your confusion about the Banking and Expense tab, @robbin-taylor.

The Banking tab is where you connect your accounts and download transactions. QuickBooks Online (QBO) offers two ways to set up bank accounts for bank feeds- Direct Connect/Connect bank and  WebConnect/Upload transactions (please see screenshot below). Whichever of these you choose, you'll save more time because you won't be entering all those transactions by hand. You’ll be downloading them from your financial institutions, which means you’ll be decreasing the introduction of errors into your books.

While the Expense tab is where you manage your general business expenses such as rent, office supplies, and more. The Expense tab will be updated with transactions that you've categorized in the Banking section, or anytime you manually create an expense transaction. However, if it's your first time setting up your QBO, you might notice that you can as well see the Connect bank option (see screenshot below). From there, you can also link your bank. 

After you are connected, QuickBooks automatically downloads the last 90 days of bank data. You choose whether to add transactions as new items or match them up with items you already entered in QBO. Doing these updates not only your expenses but also your sales depending on what entries you added or categorized.

You can check these links for your reference:

• Connect bank and credit card accounts to QBO
• Manually upload transactions into QBO
• Enter, edit, or delete expenses in QBO

I'm always here if you have other questions or concerns. Have a good one, @robbin-taylor.

I appreciated the response.  By reading through it I did get my question answered.  And I appreciated the depth of the answer, but I had to go back and figure out if the Expenses tab was already connected. So in my case a simple "it comes from the bank if your expenses are linked to the bank" would have been what I actually needed, then followed up with "this is how you find out".