The key to getting bounces sent to the right account is the Email Domain of the Mail Account that you've configured for Bounce Processing. The emails that are sent out use that domain for the "envelope" from address (which is not the one you see as sender or recipient, but the one that's hidden in the smtp conversation between machines).
As more detailed explanation is here: https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/
In summary:
The "envelope" From address for a civi message is built uniquely per message and recipient as "[email protected]". When it bounces, the bounce processing will process it based on that from address to determine what to do with it.
So your bounce domain is usually NOT your from address domain, it's a special domain that can handle bounces. You normally would not want to do this yourself, though it's been done and there are instructions to do it using specially setup gmail addresses using some more involved addressing magic.