Is it possible to provide internal notes in the invoices, bills, etc?

I can show you how to add internal notes to your transactions, @EliteEden.

You can add internal notes to the memo field when creating Receive Payment, Delayed Credit, Delayed Charge, Expense, Check, Bill, Bill Payment, Purchase Order, Vendor Credit, Credit Card Credit, Check, Bank Deposit, Transfer, and Journal Entry transactions.  

Adding notes to the Memo box won't be seen by your customers or vendors. I can see how this feature would be a great addition to the current features that we have if we apply it to all our transactions including invoices and estimates, so I'll take note of it. I'd also recommend you visiting our Blog to learn what new updates and features we have. 

You can check this article for more information: When the memo field will and will not print.

See the following article for more information about QuickBooks Online file size and feature limits. 

I'm happy to help some more if you have other questions about adding notes to your transactions in QuickBooks Online (QBO). Have a great rest of the day.

@MJoy_D thanks for sharing these info. Notes in the memo for invoices and payments are shown on the statements.