Specifying expense/income account for transactions

QBO (Web Connect) is the format downloaded from a bank, so the bank does not know how you categorize transactions. This is done after QBO import in Quickbooks on the Bank Feeds form. Quickbooks remembers your choice for an expense/income account and selects it next time it sees the payee name.

If you want to specify expense/income account for your transactions not in Quickbooks, you have to use IIF format. IIF format is the low-level format. What it means that Quickbooks imports IIF files without an interface, so you have provided expense/income accounts and vendor names exactly as you have in Quickbooks. It should be if you are creating your CSV/Excel files, but for downloaded CSV files from your bank, you have adjusted transactions first. CSV2IIF at https://www.propersoft.net/csv2iif/ does the conversion from CSV to IIF, and it maps a "Category" column to expense/income account for each transaction.