I know where to find the attachments, d-g-affordableco.
Even if you delete or make a customer inactive, the associated attachments will still be available in QuickBooks Online. You can see them through the Attachments page.
You'll see under the LINKS column that the attachment is still associated a deleted customer.
To know more about attachments, you can visit this link: Attachments in QuickBooks Online.
We'll be right here if you have additional queries. Have a great day!