If I have added a bank account, will this automatically be the account a payee pays into? Ie. do I need bank details on the invoice?
When you connect a bank account to QuickBooks Online, that's only for getting the transactions into your QuickBooks company for the purposes of recording invoices as paid, or reconciling the bank account.
You will have to put bank details on your invoices to ensure that you get paid. Once you get paid, that transaction will feed into your QuickBooks company so you can match that to the open invoice and therefore mark it as paid.