For *.qbo files, use the following reference
https://quickbooks.intuit.com/learn-support/en-us/help-article/bank-transactions/upload-transactions...
For *.iif files, you have to open the files in Excel and save as *.csv files. Then you will need to use a 3rd party converter or importer tool to proceed.
To import bank statements in csv format, use this importer tool
https://transactionpro.grsm.io/qbo
Otherwise, you have to convert *.csv files into *.qbo files, use a csv2qbo converter tool
https://www.moneythumb.com/?ref=110
